56 results on '"Marco Laurati"'
Search Results
2. Effect of Composition and Freeze-Thaw on the Network Structure, Porosity and Mechanical Properties of Polyvinyl-Alcohol/Chitosan Hydrogels
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Fernando Soto-Bustamante, Gavino Bassu, Emiliano Fratini, and Marco Laurati
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hydrogel ,cryogel ,poly (vinyl alcohol) ,chitosan ,porosity ,network structure ,Science ,Chemistry ,QD1-999 ,Inorganic chemistry ,QD146-197 ,General. Including alchemy ,QD1-65 - Abstract
We report the synthesis and characterization of poly (vinyl alcohol) (PVA)/Chitosan (CT) cryogels for applications involving the uptake and entrapment of particulate and bacterial colonies. In particular, we systematically investigated the network and pore structures of the gels as a function of CT content and for different freeze-thaw times, combining Small Angle X-Ray Scattering (SAXS), Scanning Electron Microscopy (SEM), and confocal microscopy. The nanoscale analysis obtained from SAXS shows that while the characteristic correlation length of the network is poorly affected by composition and freeze-thaw time, the characteristic size of heterogeneities associated with PVA crystallites decreases with CT content. SEM investigation evidences a transition to a more homogeneous network structure induced by the incorporation of CT that progressively builds a secondary network around the one formed by PVA. A detailed analysis of confocal microscopy image stacks allows to characterize the 3D porosity of the samples, revealing a significantly asymmetric shape of the pores. While the average volume of single pores increases with increasing CT content, the overall porosity remains almost unchanged as a result of the suppression of smaller pores in the PVA network with the progressive incorporation of the more homogeneous CT network. Increasing the freezing time in the FT cycles also results in a decrease of porosity, which can be associated with a growth in the crosslinking of the network due to PVA crystallization. The linear viscoelastic moduli measured by oscillatory rheology show a qualitatively comparable frequency-dependent response in all cases, with a moderate reduction with increasing CT content. This is attributed to changes in the structure of the strands of the PVA network.
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- 2023
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3. Reciprocal Space Study of Brownian Yet Non-Gaussian Diffusion of Small Tracers in a Hard-Sphere Glass
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Matteo Brizioli, Tatjana Sentjabrskaja, Stefan U. Egelhaaf, Marco Laurati, Roberto Cerbino, and Fabio Giavazzi
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non-Gaussian diffusion ,differential dynamic microscopy ,colloidal glasses ,diffusion in crowded environments ,quantitative microscopy ,diffusing diffusivity ,Physics ,QC1-999 - Abstract
The simultaneous presence of normal (Brownian) diffusion and non-Gaussian statistics of particle displacements has been identified as a recurring motif for a broad spectrum of physical and biological systems. While not yet fully understood, it is generally accepted that a key ingredient for observing this Brownian yet non-Gaussian (BNG) diffusion is that the environment hosting the particles appears stationary and homogenous on the small length and time scales, while displaying significant fluctuations on larger distances and/or longer time scales. To date, most of the experimental studies on systems displaying BNG diffusion have been performed in direct space, usually via a combination of optical microscopy and particle tracking to quantify the particle’s self-diffusion. Here, we demonstrate that a reciprocal space analysis of the density fluctuations caused by the particle motion as a function of the wave vector enables the investigation of BNG diffusion in situations where single-particle tracking is impossible. To accomplish this aim, we use confocal differential dynamic microscopy (ConDDM) to study the BNG dynamics of diluted sub-resolution tracers diffusing in a glassy matrix of larger hard spheres. We first elucidate the nontrivial connection between the tracer self-diffusion and collective relaxation of the resulting density fluctuations. We find that the experimentally determined intermediate scattering functions are in excellent agreement with the recent predictions of a “diffusing diffusivity” model of BNG diffusion, whose analytical predictions are available only in reciprocal space. Our results show that studying BNG diffusion in reciprocal space can be an invaluable strategy to access the fast, anomalous dynamics occurring at very small scales in crowded environments.
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- 2022
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4. Anomalous dynamics of intruders in a crowded environment of mobile obstacles
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Tatjana Sentjabrskaja, Emanuela Zaccarelli, Cristiano De Michele, Francesco Sciortino, Piero Tartaglia, Thomas Voigtmann, Stefan U. Egelhaaf, and Marco Laurati
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Science - Abstract
The classical Lorentz gas model is widely used to describe constrained transport, but its assumption of an immobile environment is not applicable to many biological and industrial processes. Here, the authors show that the mobility of the matrix induces anomalous, logarithmic dynamics of the confined particles.
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- 2016
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5. Concentration and temperature dependent interactions and state diagram of dispersions of copolymer microgels
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José Ruiz-Franco, Rodrigo Rivas-Barbosa, Mayra A. Lara-Peña, José R. Villanueva-Valencia, Angel Licea-Claverie, Emanuela Zaccarelli, and Marco Laurati
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Life Science ,General Chemistry ,Condensed Matter Physics - Abstract
We investigate by means of small angle neutron scattering experiments and numerical simulations the interactions and inter-particle arrangements of concentrated dispersions of copolymer poly(N-isopropylacrylamide)-poly(ethylene glycol methyl ether methacrylate) (PNIPAM-PEGMA) microgels across the volume phase transition (VPT). The scattering data of moderately concentrated dispersions are accurately modeled at all temperatures by using a star polymer form factor and static structure factors calculated from the effective potential obtained from simulations. Interestingly, for temperatures below the VPT temperature (VPTT), the radius of gyration and blob size of the particles significantly decrease with increasing the effective packing fraction in the non-overlapping regime. This is attributed to the presence of charges in the system associated with the use of an ionic initiator in the synthesis. Simulations using the experimentally corroborated interaction potential are used to explore the state diagram in a wide range of effective packing fractions. Below and slightly above the VPTT, the system undergoes an arrest transition mainly driven by the soft repulsion between the particles. Only well above the VPTT the system is found to phase separate before arresting. Our results highlight the versatility and potential of copolymer PNIPAM-PEGMA microgels to explore different kinds of arrested states balancing attraction and repulsion by changing temperature and packing fraction.
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- 2023
6. Clusters in colloidal dispersions with a short-range depletion attraction: Thermodynamic identification and morphology
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Fernando, Soto-Bustamante, Néstor E, Valadez-Pérez, Yun, Liu, Ramón, Castañeda-Priego, and Marco, Laurati
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Biomaterials ,Colloid and Surface Chemistry ,Thermodynamics ,Computer Simulation ,Colloids ,Gels ,Porosity ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials - Abstract
Particle aggregation is ubiquitous for many colloidal systems, and drives the phase separation or the formation of materials with a highly heterogeneous large-scale structure, such as gels, porous media and attractive glasses. While the macroscopic properties of such materials strongly depend on the shape and size of these particle aggregates, the morphology and underlining aggregation physical mechanisms are far from being fully understood. Recently, it has been proposed that for reversible colloidal aggregation, the cluster morphology in the case of colloids interacting with short-range attractive forces is determined by a single variable, namely, the reduced second virial coefficient, BWe examined this proposal by performing confocal microscopy experiments and computer simulations on a large collection of short-ranged attractive colloidal systems with different values of the attraction strength and range.We show that in all cases a connection between the colloidal cluster morphology and B
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- 2022
7. Tuning the Effective Interactions between Spherical Double-Stranded DNA Brushes
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Ivany C. Romero-Sanchez, Laura E. Castellano, and Marco Laurati
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Inorganic Chemistry ,Polymers and Plastics ,Organic Chemistry ,Materials Chemistry - Published
- 2022
8. Link between Morphology, Structure, and Interactions of Composite Microgels
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Rodrigo Rivas-Barbosa, José Ruiz-Franco, Mayra A. Lara-Peña, Jacopo Cardellini, Angel Licea-Claverie, Fabrizio Camerin, Emanuela Zaccarelli, and Marco Laurati
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soft matter ,Polymers and Plastics ,microgels, colloids, form factor, volume phase transitions, interactions ,Organic Chemistry ,FOS: Physical sciences ,molecular dynamics simulations ,effective interactions ,Condensed Matter - Soft Condensed Matter ,colloids ,microgels ,pnipam ,small-angle scattering ,Inorganic Chemistry ,Materials Chemistry ,Life Science ,Soft Condensed Matter (cond-mat.soft) ,Physical Chemistry and Soft Matter - Abstract
We combine small angle scattering experiments and simulations to investigate the internal structure and interactions of composite Poly(N-isopropylacrylamide)-Poly(ethylene glycol) (PNIPAM-PEG) microgels. At low temperatures the experimentally determined form factors and the simulated density profiles indicate a loose internal particle structure with an extended corona, that can be modeled as a star-like object. Increasing temperature across the volumetric phase transition, the form factor develops an inflection which, using simulations, is interpreted as arising from a configuration in which PEG chains are incorporated in the interior of the PNIPAM network. In this configuration a peculiar density profile characterized by two dense, separate regions is observed, at odds with configurations in which the PEG chains reside on the surface of the PNIPAM core. The conformation of the PEG chains have also profound effects on the interparticle interactions: While chains on the surface reduce the solvophobic attractions typically experienced by PNIPAM particles at high temperatures, PEG chains inside the PNIPAM network shift the onset of attractive interaction at even lower temperatures. Our results show that by tuning the morphology of the composite microgels we can qualitatively change both their structure and their mutual interactions, opening the way to explore new collective behaviors of these objects., Manuscript 35 pages, 6 figures; Supplementary material 4 pages, 3 figures
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- 2022
9. Ingestion of microplastics and textile cellulose particles by some meiofaunal taxa of an urban stream
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Tiziana Di Lorenzo, Serena Benedetta Cabigliera, Tania Martellini, Marco Laurati, David Chelazzi, Diana Maria Paola Galassi, and Alessandra Cincinelli
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Fluorescence microscopy ,Focal plane array detectors ,Functional traits ,Metazoan meiofauna ,Nile red ,μFTIR ,Animals ,Microplastics ,Plastics ,Ecosystem ,Cellulose ,Textiles ,Eating ,Environmental Monitoring ,Water Pollutants, Chemical ,Copepoda ,Environmental Engineering ,Health, Toxicology and Mutagenesis ,Chemical ,Environmental Chemistry ,Water Pollutants ,Public Health, Environmental and Occupational Health ,General Medicine ,General Chemistry ,Pollution - Abstract
Microplastics (MPs) and textile cellulose are globally pervasive pollutants in freshwater. In-situ studies assessing the ingestion of MPs by freshwater meiofauna are few. Here, we evaluated MP and textile cellulose ingestion by some meiofaunal taxa and functional guilds of a first-order stream in the city of Florence (Italy) by using a tandem microscopy approach (fluorescence microscopy and μFTIR). The study targeted five taxa (nematodes, oligochaetes, copepods, ephemeropterans and chironomids), three feeding (scrapers, deposit-feeders, and predators), and three locomotion (crawlers, burrowers, and swimmers) guilds. Fluorescent particles related to both MPs and textile cellulose resulted in high numbers in all taxa and functional guilds. We found the highest number of particles in nematodes (5200 particles/ind.) and deposit-feeders (1693 particles/ind.). Oligochaetes and chironomids (burrowers) ingested the largest particles (medium length: 28 and 48 μm, respectively), whereas deposit-feeders ingested larger particles (medium length: 26 μm) than scrapers and predators. Pellets were abundant in all taxa, except for Chironomidae. Textile cellulose fibers were present in all taxa and functional guilds, while MP polymers (EVA, PET, PA, PE, PE-PP) differed among taxa and functional guilds. In detail: EVA and PET particles were found only in chironomids, PE particles occurred in chironomids, copepods and ephemeropterans, PA particles were found in all taxa except in nematodes, whereas particles made of PE-PP blend occurred in oligochaetes and copepods. Burrowers and deposit-feeders ingested EVA, PET, PA, PE and PE-PP, while crawlers and scrapers ingested PE and PA. Swimmers and predators ingested PE, PA and PE-PP. Our findings suggest a pervasive level of plastic and textile cellulose pollution consistent with an urban stream which propagates in the meiofaunal assemblage of the stream ecosystem.
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- 2023
10. Microgel dynamics within the 3D porous structure of transparent PEG hydrogels
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Gavino, Bassu, Marco, Laurati, and Emiliano, Fratini
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Microgels ,Colloid and Surface Chemistry ,Biocompatible Materials ,Hydrogels ,Surfaces and Interfaces ,General Medicine ,Physical and Theoretical Chemistry ,Porosity ,Polyethylene Glycols ,Biotechnology - Abstract
We report an investigation on the effects of the confinement imposed by application-relevant poly(ethylene glycol) (PEG) hydrogel matrices with controlled porosity on the dynamics of soft microgels. Through a detailed characterization of the internal structure of the hydrogels at the nano and microscale, we were able to link the microgel dynamics, measured by particle tracking, to the 3D geometrical confinement imposed by the porous matrices. PEG hydrogels with a high degree of transparency and tunable pore sizes and volume fractions were obtained using freeze-thawing. We found that the porosity of the hydrogel networks is characterized by elongated channels having asymmetric sections, with the average size decreasing from about 7 to about 2 particle diameters, and the size distribution becoming narrower with increasing PEG content in the pre-reaction mixture. The microgel dynamics slowdown and change from diffusive to sub-diffusive as a result of the increasing confinement. The observed decrease in diffusivity is consistent with models of diffusion in cylindrical pores and can be attributed to hydrodynamic and steric effects in addition to geometrical constriction. A dependence of the effective diffusion coefficient on the pore volume fraction, which is unusually pronounced, suggests the presence of microgel-hydrogel interactions. Our results demonstrate that a detailed characterization of the 3D geometry of the porous network is of primary importance for the understanding of transport properties in complex, random porous media.
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- 2023
11. Contributors
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Samiul Amin, Paolo Bergese, Debora Berti, Wuge H. Briscoe, Italo Colombo, Julian Eastoe, Stefania Federici, Luigi Gentile, Stefan Kudera, Marco Laurati, Liberato Manna, Helena Mateos, Ulf Olsson, Luigi Paduano, Gerardo Palazzo, and Rico F. Tabor
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- 2022
12. Ingestion of Microplastics and Textile Cellulose Particles by Some Meiofaunal Taxa of an Urban Stream
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Tiziana Di Lorenzo, Serena Benedetta Cabigliera, Tania Martellini, Marco Laurati, David Chelazzi, and Alessandra Cincinelli
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History ,Polymers and Plastics ,Business and International Management ,Industrial and Manufacturing Engineering - Published
- 2022
13. Structure and dynamics: Static scattering of radiation and optical correlation techniques
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Debora Berti and Marco Laurati
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Materials science ,Scattering ,Dynamics (mechanics) ,Structure (category theory) ,Radiation ,Optical correlation ,Computational physics - Published
- 2022
14. i-Rheo: determining the linear viscoelastic moduli of colloidal dispersions from step-stress measurements
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Rodrigo Rivas-Barbosa, Marco Laurati, Manlio Tassieri, and M. A. Escobedo-Sánchez
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Materials science ,media_common.quotation_subject ,General Physics and Astronomy ,02 engineering and technology ,Mechanics ,021001 nanoscience & nanotechnology ,Inertia ,01 natural sciences ,Viscoelasticity ,Moduli ,symbols.namesake ,Fourier transform ,Creep ,Rheology ,0103 physical sciences ,symbols ,Physical and Theoretical Chemistry ,Elasticity (economics) ,010306 general physics ,0210 nano-technology ,Rheology, Colloids, Step Stress, Linear Viscoelasticity ,Complex fluid ,media_common - Abstract
We report on the application of a Fourier transform-based method, 'i-Rheo', to evaluate the linear viscoelastic moduli of hard-sphere colloidal dispersions, both in the fluid and glass states, from a direct analysis of raw step-stress (creep) experimental data. We corroborate the efficacy of i-Rheo by comparing the outputs of creep tests performed on homogenous complex fluids to conventional dynamic frequency sweeps. A similar approach is adopted for a number of colloidal suspensions over a broad range of volume fractions. For these systems, we test the limits of the method by varying the applied stress across the materials' linear and non-linear viscoelastic regimes, and we show that the best results are achieved for stress values close to the upper limit of the materials' linear viscoelastic regime, where the signal-to-noise ratio is at its highest and the non-linear phenomena have not appeared yet. We record that, the range of accessible frequencies is controlled at the higher end by the relative weight between the inertia of the instrument and the elasticity of the complex material under investigation; whereas, the lowest accessible frequency is dictated by the extent of the materials' linear viscoelastic regime. Nonetheless, despite these constrains, we confirm the effectiveness of i-Rheo for gaining valuable information on the materials' linear viscoelastic properties even from 'creep ringing' data, confirming its potency and general validity as an accurate method for determining the material's rheological behaviour for a variety of complex systems.
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- 2020
15. Potential-invariant network structures in Asakura-Oosawa mixtures with very short attraction range
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Ramón Castañeda-Priego, Néstor E. Valadez-Pérez, Fernando Soto Bustamante, and Marco Laurati
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chemistry.chemical_classification ,Materials science ,General Physics and Astronomy ,Polymer ,Attraction ,Network formation ,Colloid ,Distribution function ,chemistry ,Chemical physics ,Volume fraction ,Cluster (physics) ,Radius of gyration ,Physical and Theoretical Chemistry - Abstract
We systematically investigated the structure and aggregate morphology of gel networks formed by colloid–polymer mixtures with a moderate colloid volume fraction and different values of the polymer–colloid size ratio, always in the limit of short-range attraction. Using the coordinates obtained from confocal microscopy experiments, we determined the radial, angular, and nearest-neighbor distribution functions together with the cluster radius of gyration as a function of size ratio and polymer concentration. The analysis of the structural correlations reveals that the network structure becomes increasingly less sensitive to the potential strength with the decreasing polymer–colloid size ratio. For the larger size ratios, compact clusters are formed at the onset of network formation and become progressively more branched and elongated with increasing polymer concentration/attraction strength. For the smallest size ratios, we observe that the aggregate structures forming the gel network are characterized by similar morphological parameters for different values of the size ratio and the polymer concentration, indicating a limited evolution of the gel structure with variations of the parameters that determine the interaction potential between colloids.
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- 2021
16. Blunt-end driven re-entrant ordering in quasi two-dimensional dispersions of spherical DNA brushes
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Ivany Romero-Sanchez, Ilian Pihlajamaa, Natasa Adžić, Laura E. Castellano, Emmanuel Stiakakis, Christos N. Likos, Marco Laurati, Soft Matter and Biological Physics, and Non-Equilibrium Soft Matter
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order transitions ,Static Electricity ,General Engineering ,General Physics and Astronomy ,FOS: Physical sciences ,DNA ,Condensed Matter - Soft Condensed Matter ,polyelectrolytes ,colloids ,ddc:540 ,blunt-ends ,Soft Condensed Matter (cond-mat.soft) ,General Materials Science ,blunt-ends colloids dna polyelectrolytes order transitions - Abstract
We investigate the effects of crowding on the conformations and assembly of confined, highly charged, and thick polyelectrolyte brushes in the osmotic regime. Particle tracking experiments on increasingly dense suspensions of colloids coated with ultra-long double stranded DNA (dsDNA) fragments reveal non-monotonic particle shrinking, aggregation and re-entrant ordering. Theory and simulations show that shrinking is induced by the osmotic pressure exerted by the counterions absorbed in neighbor brushes, while aggregation and re-entrant ordering are the effect of a short-range attraction competing with the electrostatic repulsion. Blunt-end interactions between dsDNA fragments of neighboring brushes are responsible for the attraction and can be tuned by inducing free-end backfolding through the addition of monovalent salt. Our results show that base stacking is a mode parallel to hybridization to steer colloidal assembly, in which attractions can be fine-tuned through salinity and, potentially, grafting density and temperature., Comment: 56 pages, 11 figures
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- 2021
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17. Colloidal and polymeric contributions to the yielding of dense microgel suspensions
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M.A. Lara-Peña, A. Licea-Claverie, Marco Laurati, and I. Zapata-González
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Materials science ,Yield (engineering) ,Colloid, Microgel, Rheology, Yielding ,02 engineering and technology ,Dynamic mechanical analysis ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Viscoelasticity ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Condensed Matter::Soft Condensed Matter ,Biomaterials ,Shear rate ,Shear (sheet metal) ,Colloid and Surface Chemistry ,Rheology ,Chemical physics ,Volume fraction ,Particle ,0210 nano-technology - Abstract
Hypothesis Soft microgel colloids can be densely packed since particle networks can compress and interpenetrate. This evolution of the particle’s internal structure associated with packing is expected to determine the linear viscoelastic properties and the yielding behavior of dense suspensions of microgel colloids. Experiments We investigated the volume fraction-dependent linear and non-linear rheological response of suspensions of soft core-shell particles formed by a poly(N-isopropylacrylamide) (PNIPAM) microgel core and a thin poly(ethylene glycol) (PEG) shell. Findings The linear viscoelasticity of suspensions reveals a transition from a fluid to a jammed glass state. Increasing volume fraction within the jammed state, the linear storage modulus and the yield stress show distinct regimes associated with the evolution of particle contacts, which involve progressive compression and interpenetration of the shell and core. The yielding of jammed suspensions occurs in two-steps: At small strains jammed cages are rearranged, while full disentanglement of interpenetrating networks only occurs at large deformations and results in fluidization. Yield strains and stresses increase with increasing shear rate or frequency, suggesting a progressive dominance of the timescale associated with shear over that associated with the internal dynamics of the system.
- Published
- 2020
18. Rheology of colloidal and metallic glass formers
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Sebastian Fritschi, Stefan U. Egelhaaf, Christian P. Amann, Marco Laurati, Matthias Krüger, Konrad Samwer, Kevin J. Mutch, Thomas Voigtmann, and Miriam Siebenbürger
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Materials science ,Colloidal suspensions ,Metallic alloys ,Rheology ,Polymers and Plastics ,02 engineering and technology ,01 natural sciences ,Moduli ,colloidal suspensions ,Colloid ,Colloid and Surface Chemistry ,0103 physical sciences ,Materials Chemistry ,ddc:530 ,Soft matter ,metallic alloys ,Physical and Theoretical Chemistry ,010306 general physics ,Amorphous metal ,Institut für Materialphysik im Weltraum ,021001 nanoscience & nanotechnology ,Condensed Matter::Soft Condensed Matter ,Chemical physics ,Particle ,rheology ,Deformation (engineering) ,0210 nano-technology ,Glass transition - Abstract
Colloidal hard-sphere suspensions are convenient experimental models to understand soft matter, and also by analogy the structural-relaxation behavior of atomic or small-molecular fluids. We discuss this analogy for the flow and deformation behavior close to the glass transition. Based on a mapping of temperature to effective hard-sphere packing, the stress–strain curves of typical bulk metallic glass formers can be quantitatively compared with those of hard-sphere suspensions. Experiments on colloids give access to the microscopic structure under deformation on a single-particle level, providing insight into the yielding mechanisms that are likely also relevant for metallic glasses. We discuss the influence of higher-order angular signals in connection with non-affine particle rearrangements close to yielding. The results are qualitatively explained on the basis of the mode-coupling theory. We further illustrate the analogy of pre-strain dependence of the linear-elastic moduli using data on PS-PNiPAM suspensions.
- Published
- 2020
19. AFM investigation of the influence of ethanol absorption on the surface structure and elasticity of polyamides
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Alejandro Martinez-Borquez, Jaime Julio Cervera-Moreno, Paul Sotta, Marco Laurati, Laboratoire Polymères et Matériaux Avancés (LPMA), Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS), and Universidad de Guanajuato, División e Ingenierías-León, Guanajuato, Mexico
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Materials science ,General Chemical Engineering ,General Physics and Astronomy ,Young's modulus ,02 engineering and technology ,Surface finish ,010402 general chemistry ,01 natural sciences ,law.invention ,symbols.namesake ,law ,General Materials Science ,Composite material ,Crystallization ,Elasticity (economics) ,[PHYS.COND]Physics [physics]/Condensed Matter [cond-mat] ,ComputingMilieux_MISCELLANEOUS ,General Environmental Science ,Atomic force microscopy ,technology, industry, and agriculture ,General Engineering ,Plasticizer ,[CHIM.MATE]Chemical Sciences/Material chemistry ,021001 nanoscience & nanotechnology ,0104 chemical sciences ,Polyamide ,symbols ,General Earth and Planetary Sciences ,AFM, polyamides, polar solvent, plasticization ,0210 nano-technology ,Glass transition - Abstract
We study the variations of surface structure and elasticity of different types of Nylon during absorption of ethanol using AFM surface topography and AFM micro-mechanics. We observe that ethanol absorption induces significant changes in the surface structure, in particular an increase of roughness as well as the presence of local damage. All investigated Nylons soften by a factor 1.5–2.5 as a consequence of ethanol absorption. The Young modulus of the samples as a function of immersion time shows a complex, non-monotonic behavior with a minimum at intermediate immersion times. This behavior results from an interplay between the reduction of the glass transition temperature $$T_g$$ , typically observed in bulk, and surface-specific effects such as the onset of surface crystallisation and degradation.
- Published
- 2019
20. Different routes into the glass state for soft thermo-sensitive colloids
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Pedro E. Ramírez-González, Tim Still, Valeria Piazza, Magdaleno Medina-Noyola, Rodrigo Rivas-Barbosa, Patricia Mendoza-Méndez, Marco Laurati, and Edilio Lázaro-Lázaro
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Materials science ,Particle number ,02 engineering and technology ,General Chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Glasses, Soft Colloids, Rheology, Structure ,Condensed Matter::Soft Condensed Matter ,Colloid ,Rheology ,Chemical physics ,Generalized langevin equation ,0103 physical sciences ,Gradual increase ,010306 general physics ,0210 nano-technology - Abstract
We report an experimental and theoretical investigation of glass formation in soft thermo-sensitive colloids following two different routes: a gradual increase of the particle number density at constant temperature and an increase of the radius in a fixed volume at constant particle number density. Confocal microscopy experiments and the non-equilibrium self-consistent generalized Langevin equation (NE-SCGLE) theory consistently show that the two routes lead to a dynamically comparable state at sufficiently long aging times. However, experiments reveal the presence of moderate but persistent structural differences. Successive cycles of radius decrease and increase lead instead to a reproducible glass state, indicating a suitable route to obtain rejuvenation without using shear fields.
- Published
- 2018
21. Investigation of moderately turbid suspensions by heterodyne near field scattering
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Stefan U. Egelhaaf, Marco Laurati, M. A. Escobedo-Sánchez, and L. F. Rojas-Ochoa
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Heterodyne ,Physics ,business.industry ,Scattering ,Multiangle light scattering ,Near and far field ,02 engineering and technology ,General Chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Light scattering ,Computational physics ,Optics ,0103 physical sciences ,Particle ,Static light scattering ,Soft matter ,010306 general physics ,0210 nano-technology ,business - Abstract
Light scattering has proven to be a very powerful technique to characterize soft matter systems. However, many samples are turbid and hence suffer from multiple scattering which can affect the signal considerably. Multiple scattering can be reduced by diluting the sample or changing the solvent, but often this alters the sample and hence is precluded. Here we study the dynamics of a model system. In particular, we investigate the effects of moderate multiple scattering on small-angle heterodyne near field scattering (HNFS). Varying the particle concentration and size we change the degree of multiple scattering, which is quantified by the transmission of light. In dependence of the degree of multiple scattering, we analyze the statistical properties of the HNFS signal, which is the difference between two intensity patterns separated by a delay time. The distribution of intensity differences follows a Gaussian distribution if single scattering dominates and a Laplace distribution in the presence of extreme multiple scattering. We also investigate the effects of multiple scattering on the measured intermediate scattering function and the hydrodynamic radius of the particles. Reliable data are obtained for sample transmissions down to about 0.7. This is confirmed by a comparison with results from a far field cross-correlation instrument that suppresses multiple scattering contributions. Therefore, HNFS represents a technically simple but powerful method to investigate samples that are moderately multiple scattering.
- Published
- 2017
22. Model-Free Rheo-AFM Probes the Viscoelasticity of Tunable DNA Soft Colloids
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Jose A. Moreno-Guerra, Ivany C. Romero‐Sánchez, Emmanuel Stiakakis, Marco Laurati, Alejandro Martinez-Borquez, and Manlio Tassieri
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endocrine system ,AFM ,brush ,colloids ,dsDNA ,polyelectrolytes ,viscoelasticity ,Materials science ,Time Factors ,02 engineering and technology ,010402 general chemistry ,Microscopy, Atomic Force ,01 natural sciences ,Viscoelasticity ,Biomaterials ,Rheology ,Elastic Modulus ,General Materials Science ,Colloids ,Exponential decay ,Softening ,chemistry.chemical_classification ,Quantitative Biology::Biomolecules ,Viscosity ,Relaxation (NMR) ,General Chemistry ,Dynamic mechanical analysis ,DNA ,021001 nanoscience & nanotechnology ,Polyelectrolyte ,Elasticity ,0104 chemical sciences ,Condensed Matter::Soft Condensed Matter ,chemistry ,Chemical physics ,ddc:540 ,Salts ,Counterion ,0210 nano-technology ,Biotechnology - Abstract
Atomic force microscopy rheological measurements (Rheo‐AFM) of the linear viscoelastic properties of single, charged colloids having a star‐like architecture with a hard core and an extended, deformable double‐stranded DNA (dsDNA) corona dispersed in aqueous saline solutions are reported. This is achieved by analyzing indentation and relaxation experiments performed on individual colloidal particles by means of a novel model‐free Fourier transform method that allows a direct evaluation of the frequency‐dependent linear viscoelastic moduli of the system under investigation. The method provides results that are consistent with those obtained via a conventional fitting procedure of the force‐relaxation curves based on a modified Maxwell model. The outcomes show a pronounced softening of the dsDNA colloids, which is described by an exponential decay of both the Young's and the storage modulus as a function of the salt concentration within the dispersing medium. The strong softening is related to a critical reduction of the size of the dsDNA corona, down to ≈70% of its size in a salt‐free solution. This can be correlated to significant topological changes of the dense star‐like polyelectrolyte forming the corona, which are induced by variations in the density profile of the counterions. Similarly, a significant reduction of the stiffness is obtained by increasing the length of the dsDNA chains, which we attribute to a reduction of the DNA density in the outer region of the corona.
- Published
- 2019
23. Binary colloidal glasses: linear viscoelasticity and its link to the microscopic structure and dynamics
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George Petekidis, Tatjana Sentjabrskaja, Thomas Voigtmann, Marco Laurati, Stefan U. Egelhaaf, and Alan R. Jacob
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Materials science ,media_common.quotation_subject ,02 engineering and technology ,010402 general chemistry ,01 natural sciences ,Asymmetry ,Viscoelasticity ,Moduli ,Rheology ,shear modulus ,glass transition ,Soft matter ,media_common ,General Chemistry ,linear viscoelasticity ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,0104 chemical sciences ,Condensed Matter::Soft Condensed Matter ,Chemical physics ,Einstein relation ,binary mixtures ,Brownian dynamics ,rheology ,0210 nano-technology ,Glass transition - Abstract
We study the relation between the microscopic structure and dynamics and the macroscopic rheological response of glass-forming colloidal suspensions, namely binary colloidal hard-sphere mixtures with large size asymmetry (1 : 5) that span a large range of mixture compositions close to the glass transition. The dynamical shear moduli are measured by oscillatory rheology and the structure and dynamics on the single-particle level by confocal microscopy. The data are compared with Brownian Dynamics simulations and predictions from mode-coupling theory based on the Percus-Yevick approximation. Experiments, simulations and theory consistently observe a strong decrease of the intermediate-frequency mechanical moduli combined with faster dynamics at intermediate mixing ratios and hence a non-monotonic dependence of these parameters but a localization of the large particles which decreases monotonically as the fraction of small particles is increased. We find that the Generalized-Stokes Einstein relation applied to the mean square displacements of the two components leads to a reasonable estimate of the shear moduli of the mixtures and hence links the rheological response to the particle dynamics which in turn reflects the microscopic structure.
- Published
- 2019
24. Effect of size disparity on the structure and dynamics of the small component in concentrated binary colloidal mixtures
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E. Martinez-Sotelo, Marco Laurati, and M. A. Escobedo-Sánchez
- Subjects
Materials science ,010304 chemical physics ,Component (thermodynamics) ,Dynamics (mechanics) ,Mixing (process engineering) ,General Physics and Astronomy ,Binary number ,010402 general chemistry ,01 natural sciences ,Colloids, Binary Mixtures, Glasses, Confocal Microscopy ,0104 chemical sciences ,Colloid ,Chemical physics ,0103 physical sciences ,Volume fraction ,SPHERES ,Physical and Theoretical Chemistry ,Glass transition - Abstract
We determined, using confocal microscopy, the structure and dynamics of the small component in concentrated binary colloidal mixtures with moderate and large size ratios and different compositions of Polymethyl methacrylate particles. We show that when increasing the content of small spheres at fixed total volume fraction, a transition in the local environment of the small particles is observed, from a mixed environment of other small and large particles to a local environment of only small particles. The transition is rather abrupt for moderate size ratios, while it becomes particularly broad for large size ratios. This can be associated with the improved ability of the small particles to pack in between the large particles for larger size ratios. The dynamics reflect the transition with an increase of the mobility observed at intermediate mixing. This increase becomes particularly pronounced for large size ratios, leading to diffusive dynamics of the small particles, in agreement with predictions of theories of the glass transition in binary hard-sphere mixtures. The composition at which the fastest dynamics are observed is apparently independent of the size ratio.We determined, using confocal microscopy, the structure and dynamics of the small component in concentrated binary colloidal mixtures with moderate and large size ratios and different compositions of Polymethyl methacrylate particles. We show that when increasing the content of small spheres at fixed total volume fraction, a transition in the local environment of the small particles is observed, from a mixed environment of other small and large particles to a local environment of only small particles. The transition is rather abrupt for moderate size ratios, while it becomes particularly broad for large size ratios. This can be associated with the improved ability of the small particles to pack in between the large particles for larger size ratios. The dynamics reflect the transition with an increase of the mobility observed at intermediate mixing. This increase becomes particularly pronounced for large size ratios, leading to diffusive dynamics of the small particles, in agreement with predictions of the...
- Published
- 2019
25. Structure of colloidal gels at intermediate concentrations: the role of competing interactions
- Author
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R. F. Capellmann, Stefan U. Egelhaaf, Marco Laurati, Néstor E. Valadez-Pérez, Benedikt Simon, and Ramón Castañeda-Priego
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Chemistry ,Monte Carlo method ,02 engineering and technology ,General Chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Electrostatics ,01 natural sciences ,Condensed Matter::Soft Condensed Matter ,Colloid ,Distribution function ,Virial coefficient ,Computational chemistry ,Chemical physics ,Yield (chemistry) ,0103 physical sciences ,Volume fraction ,Colloids, Gels, Structure, Dynamics, Confocal Microscopy, Simulation ,010306 general physics ,0210 nano-technology ,Pair potential - Abstract
Colloidal gels formed by colloid–polymer mixtures with an intermediate volume fraction (ϕc ≈ 0.4) are investigated by confocal microscopy. In addition, we have performed Monte Carlo simulations based on a simple effective pair potential that includes a short-range attractive contribution representing depletion interactions, and a longer-range repulsive contribution describing the electrostatic interactions due to the presence of residual charges. Despite neglecting non-equilibrium effects, experiments and simulations yield similar gel structures, characterised by, e.g., the pair, angular and bond distribution functions. We find that the structure hardly depends on the strength of the attraction if the electrostatic contribution is fixed, but changes significantly if the electrostatic screening is changed. This delicate balance between attractions and repulsions, which we quantify by the second virial coefficient, also determines the location of the gelation boundary.
- Published
- 2016
26. Glassy dynamics in asymmetric binary mixtures of hard-spheres
- Author
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Jorge Adrián Perera-Burgos, Gabriel Pérez-Ángel, Marco Laurati, Ramón Castañeda-Priego, Stefan U. Egelhaaf, Magdaleno Medina-Noyola, Thomas Voigtmann, Tatjana Sentjabrskaja, Patrick Laermann, Luis Fernando Elizondo-Aguilera, and Edilio Lázaro-Lázaro
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Physics ,Binary number ,FOS: Physical sciences ,Hard spheres ,Condensed Matter - Soft Condensed Matter ,01 natural sciences ,010305 fluids & plasmas ,Amorphous solid ,colloidal suspensions ,Condensed Matter::Soft Condensed Matter ,Molecular dynamics ,binary mixtures ,0103 physical sciences ,Ergodic theory ,Soft Condensed Matter (cond-mat.soft) ,SPHERES ,glass transition ,Statistical physics ,State diagram ,Glasses, Colloids, Mixtures, Dynamics ,010306 general physics ,Glass transition ,partial arrest - Abstract
The binary hard-sphere mixture is one of the simplest representations of a many-body system with competing time and length scales. This model is relevant to fundamentally understand both the structural and dynamical properties of materials, such as metallic melts, colloids, polymers and bio-based composites. It also allows us to study how different scales influence the physical behavior of a multicomponent glass-forming liquid; a question that still awaits a unified description. In this contribution, we report on distinct dynamical arrest transitions in highly asymmetric binary colloidal mixtures, namely, a single glass of big particles, in which the small species remains ergodic, and a double glass with the simultaneous arrest of both components. When the mixture approaches any glass transition, the relaxation of the collective dynamics of both species becomes coupled. In the single glass domain, spatial modulations occur due to the structure of the large spheres, a feature not observed in the two-glass domain. The relaxation of the \emph{self} dynamics of small and large particles, in contrast, become decoupled at the boundaries of both transitions; the large species always displays dynamical arrest, whereas the small ones appear arrested only in the double glass. Thus, in order to obtain a complete picture of the distinct glassy states, one needs to take into account the dynamics of both species.
- Published
- 2018
- Full Text
- View/download PDF
27. Binary colloidal glasses under transient stress- and strain-controlled shear
- Author
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Stefan U. Egelhaaf, George Petekidis, Alan R. Jacob, Tatjana Sentjabrskaja, Jan Hendricks, and Marco Laurati
- Subjects
Materials science ,Mechanical Engineering ,Rheology, Colloids, Binary Mixtures, Glasses ,Thermodynamics ,02 engineering and technology ,Hard spheres ,Strain rate ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Shear rate ,Condensed Matter::Soft Condensed Matter ,Rheology ,Creep ,Mechanics of Materials ,0103 physical sciences ,General Materials Science ,SPHERES ,Transient response ,010306 general physics ,0210 nano-technology ,Shear flow - Abstract
We investigate the yielding and transition to flow of different colloidal glasses. Using a single model system, a binary mixture of colloidal hard spheres with different compositions and size ratios, we study single, double and asymmetric glasses, which differ in the degree of mobility of the small particles and the caging mechanisms of the large spheres. The rheological response following either a step to a constant shear rate or to a constant stress (creep) is measured and the two responses are quantitatively compared. Although the same steady state of flow is observed at long times, the transient responses in strain- and stress-controlled experiments differ significantly. To achieve yielding and a steady state of flow, less time and less energy input is required if a constant strain rate is applied. Moreover, larger strain rates or stresses result in faster yielding and flow, but require more total energy input. If a constant strain rate is applied, yielding and the transition to flow depend on the pro...
- Published
- 2018
28. Size-Dependent Localization in Polydisperse Colloidal Glasses
- Author
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Kevin J. Mutch, D. Heckendorf, Marco Laurati, and Stefan U. Egelhaaf
- Subjects
Materials science ,Dispersity ,Size dependent ,General Physics and Astronomy ,02 engineering and technology ,Hard spheres ,021001 nanoscience & nanotechnology ,01 natural sciences ,Colloids, Glasses, Polydispersity, Confocal Microscopy ,Colloid ,Chemical physics ,0103 physical sciences ,Volume fraction ,Particle size ,010306 general physics ,0210 nano-technology - Abstract
We have investigated concentrated suspensions of polydisperse hard spheres and have determined the dynamics and sizes of individual particles using confocal microscopy. With increasing concentration, the dynamics of the small and large particles start to differ. The large particles exhibit slower dynamics and stronger localization. Moreover, as the particle size increases, the local volume fraction ϕ_{loc} also increases. In the glass state, the localization length significantly decreases beyond ϕ_{loc}≈0.67. This suggests a link between local crowding and dynamical heterogeneities. However dynamical arrest of subpopulations seems not directly linked to a large value of ϕ_{loc}, indicating the importance of collective effects.
- Published
- 2017
29. Long-Lived Neighbors Determine the Rheological Response of Glasses
- Author
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Kevin J. Mutch, Alessio Zaccone, P Maßhoff, Marco Laurati, Stefan U. Egelhaaf, and Apollo - University of Cambridge Repository
- Subjects
cond-mat.soft ,Materials science ,Particle number ,FOS: Physical sciences ,General Physics and Astronomy ,02 engineering and technology ,Condensed Matter - Soft Condensed Matter ,021001 nanoscience & nanotechnology ,01 natural sciences ,Viscoelasticity ,Condensed Matter::Soft Condensed Matter ,Shear rate ,Colloids, Glasses, Rheology, Confocal Microscopy ,Shear (geology) ,Rheology ,0103 physical sciences ,Volume fraction ,Soft Condensed Matter (cond-mat.soft) ,Affine transformation ,Statistical physics ,010306 general physics ,0210 nano-technology - Abstract
Glasses exhibit a liquid-like structure but a solid-like rheological response with plastic deformations only occurring beyond yielding. Thus, predicting the rheological behavior from the microscopic structure is difficult, but important for materials science. Here, we consider colloidal suspensions and propose to supplement the static structural information with the local dynamics, namely the rearrangement and breaking of the cage of neighbors. This is quantified by the mean squared nonaffine displacement and the number of particles that remain nearest neighbors for a long time, i.e. long-lived neighbors, respectively. Both quantities are followed under shear using confocal microscopy and are the basis to calculate the affine and nonaffine contributions to the elastic stress, which is complemented by the viscous stress to give the total stress. During start-up of shear, the model predicts three transient regimes that result from the interplay of affine, nonaffine and viscous contributions. Our prediction quantitatively agrees with rheological data and their dependencies on volume fraction and shear rate., 5 pages, 3 figures, Accepted for publication in Phys. Rev. Lett. Supplemental Information available as ancillary file
- Published
- 2017
30. One- and two-component colloidal glasses under transient shear
- Author
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Marco Laurati, Stefan U. Egelhaaf, and Tatjana Sentjabrskaja
- Subjects
Materials science ,General Physics and Astronomy ,Nanotechnology ,02 engineering and technology ,Mechanics ,021001 nanoscience & nanotechnology ,01 natural sciences ,Viscoelasticity ,Shear rate ,Condensed Matter::Soft Condensed Matter ,Colloid ,Rheology ,Shear (geology) ,Creep ,Rheology, Shear, Colloids, Glasses, Binary ,0103 physical sciences ,Shear stress ,General Materials Science ,Physical and Theoretical Chemistry ,010306 general physics ,0210 nano-technology - Abstract
In concentrated colloidal mixtures different caging mechanisms exist and result in different arrested states: repulsive, attractive and asymmetric glasses as well as gel-like states. We discuss their microscopic structure, dynamics and rheological response. Special attention is given to the non-linear mechanical behaviour, in particular the transient rheological response after shear is started. Steps in both, shear rate and shear stress (creep test), are considered. The macroscopic viscoelastic response is related to the microscopic structure and dynamics on the individual-particle level.
- Published
- 2017
31. Plastic rearrangements in colloidal gels investigated by LAOS and LS-Echo
- Author
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George Petekidis, Stefan U. Egelhaaf, and Marco Laurati
- Subjects
Rheology, LAOS, Colloids, Gels, Diffusing Wave Spectroscopy ,Dilatant ,Materials science ,Strain (chemistry) ,Mechanical Engineering ,02 engineering and technology ,Work hardening ,Plasticity ,Strain hardening exponent ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Light scattering ,Condensed Matter::Soft Condensed Matter ,Stress (mechanics) ,Rheology ,Mechanics of Materials ,Chemical physics ,0103 physical sciences ,General Materials Science ,010306 general physics ,0210 nano-technology - Abstract
We investigate the yielding behavior of colloid-polymer gels with intermediate volume fraction, which are subjected to large amplitude oscillatory shear, using rheology and light scattering echo (LS-Echo). Particular attention is given to the anharmonic contributions to the stress response and the characteristic timescale and extent of plastic rearrangements. Yielding is already observed at small strain amplitudes γ0≈1%, where the network of interconnected clusters starts to break up and irreversible particle rearrangements are first observed. However, only at considerably larger strain amplitudes, γ0≳100%, the network is completely disrupted and small clusters or individual particles flow. This complex yielding behavior is reflected in different regimes of in-cycle yielding, which were extracted from the analysis of the anharmonic contributions to the stress. In the range of strain amplitudes where bond breaking starts, in-cycle strain hardening and shear thickening are observed, with the strain hardenin...
- Published
- 2014
32. Start-up shear of concentrated colloidal hard spheres: Stresses, dynamics, and structure
- Author
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Nick Koumakis, Alan R. Jacob, Andrew B. Schofield, Marco Laurati, John F. Brady, A. Abdellali, George Petekidis, Kevin J. Mutch, and Stefan U. Egelhaaf
- Subjects
Materials science ,FOS: Physical sciences ,02 engineering and technology ,Péclet number ,Condensed Matter - Soft Condensed Matter ,Rheology, colloids, simulation, microscopy, glasses ,Radial distribution function ,01 natural sciences ,symbols.namesake ,0103 physical sciences ,General Materials Science ,010306 general physics ,Anisotropy ,Mechanical Engineering ,Linear elasticity ,Mechanics ,Hard spheres ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Condensed Matter::Soft Condensed Matter ,Shear (geology) ,Mechanics of Materials ,Volume fraction ,Brownian dynamics ,symbols ,Soft Condensed Matter (cond-mat.soft) ,0210 nano-technology - Abstract
The transient response of model hard sphere glasses is examined during the application of steady rate start-up shear using Brownian Dynamics (BD) simulations, experimental rheology and confocal microscopy. With increasing strain the glass initially exhibits an almost linear elastic stress increase, a stress peak at the yield point and then reaches a constant steady state. The stress overshoot has a non-monotonic dependence with Peclet number, Pe, and volume fraction, {\phi}, determined by the available free volume and a competition between structural relaxation and shear advection. Examination of the structural properties under shear revealed an increasing anisotropic radial distribution function, g(r), mostly in the velocity - gradient (xy) plane, which decreases after the stress peak with considerable anisotropy remaining in the steady-state. Low rates minimally distort the structure, while high rates show distortion with signatures of transient elongation. As a mechanism of storing energy, particles are trapped within a cage distorted more than Brownian relaxation allows, while at larger strains, stresses are relaxed as particles are forced out of the cage due to advection. Even in the steady state, intermediate super diffusion is observed at high rates and is a signature of the continuous breaking and reformation of cages under shear.
- Published
- 2016
33. i-Rheo: Measuring the materials' linear viscoelastic properties 'in a step'!
- Author
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Manlio Tassieri, D. J. Curtis, Phylip Rhodri Williams, Karl Hawkins, Jonathan M. Cooper, Dietmar Auhl, Marco Laurati, Salvatore Coppola, Andrea Scalfati, RS: FSE AMIBM, and AMIBM
- Subjects
DYNAMICS ,Gel point ,Materials science ,SUSPENSIONS ,02 engineering and technology ,01 natural sciences ,BLENDS ,Viscoelasticity ,symbols.namesake ,Natural rubber ,Rheology ,STRESS-RELAXATION ,0103 physical sciences ,Stress relaxation ,General Materials Science ,Suspension (vehicle) ,Rheology, Viscoelasticity, Fourier Tranform ,POLYETHYLENE ,SPECTROSCOPY ,010304 chemical physics ,Mechanical Engineering ,TUBE DILATION ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Condensed Matter::Soft Condensed Matter ,Fourier transform ,Mechanics of Materials ,visual_art ,symbols ,visual_art.visual_art_medium ,Polymer blend ,SHEAR-FLOW ,MICRORHEOLOGY ,0210 nano-technology ,Biological system ,BEHAVIOR - Abstract
We present a simple new analytical method for educing the materials' linear viscoelastic properties, over the widest range of experimentally accessible frequencies, from a simple step-strain measurement, without the need of preconceived models nor the idealization of real measurements. This is achieved by evaluating the Fourier transforms of raw experimental data describing both the time-dependent stress and strain functions. The novel method has been implemented into an open access executable "i-Rheo," enabling its use to a broad scientific community. The effectiveness of the new rheological tool has been corroborated by direct comparison with conventional linear oscillatory measurements for a series of complex materials as diverse as a monodisperse linear polymer melt, a bimodal blend of linear polymer melts, an industrial styrene-butadiene rubber, an aqueous gelatin solution at the gel point and a highly concentrated suspension of colloidal particles. The broadband nature of the new method and its general validity open the route to a deeper understanding of the material's rheological behavior in a variety of systems. (C) 2016 The Society of Rheology.
- Published
- 2016
34. Directed percolation identified as equilibrium pre-transition towards non-equilibrium arrested gel states
- Author
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R. F. Capellmann, Michael Schmiedeberg, Matthias Kohl, Marco Laurati, and Stefan U. Egelhaaf
- Subjects
Materials science ,Science ,education ,General Physics and Astronomy ,Network structure ,Nanotechnology ,02 engineering and technology ,01 natural sciences ,General Biochemistry, Genetics and Molecular Biology ,Article ,Quantitative Biology::Cell Behavior ,Quantitative Biology::Subcellular Processes ,0103 physical sciences ,Gel, Colloid, Percolation, Microscopy, Simulation ,Structural transition ,Soft matter ,010306 general physics ,Multidisciplinary ,General Chemistry ,Renormalization group ,021001 nanoscience & nanotechnology ,Directed percolation ,Condensed Matter::Soft Condensed Matter ,Chemical physics ,Self-assembly ,0210 nano-technology - Abstract
The macroscopic properties of gels arise from their slow dynamics and load-bearing network structure, which are exploited by nature and in numerous industrial products. However, a link between these structural and dynamical properties has remained elusive. Here we present confocal microscopy experiments and simulations of gel-forming colloid–polymer mixtures. They reveal that gel formation is preceded by continuous and directed percolation. Both transitions lead to system-spanning networks, but only directed percolation results in extremely slow dynamics, ageing and a shrinking of the gel that resembles synaeresis. Therefore, dynamical arrest in gels is found to be linked to a structural transition, namely directed percolation, which is quantitatively associated with the mean number of bonded neighbours. Directed percolation denotes a universality class of transitions. Our study hence connects gel formation to a well-developed theoretical framework, which now can be exploited to achieve a detailed understanding of arrested gels., Gels exhibit very slow dynamics, for which a structural reason remains elusive. Here, Kohl et al. show the gel formation is accompanied by a succession of continuous and directed percolation, with only the latter found to lead to the arrested dynamics.
- Published
- 2016
35. Nonlinear rheology of colloidal gels with intermediate volume fraction
- Author
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George Petekidis, Stefan U. Egelhaaf, and Marco Laurati
- Subjects
chemistry.chemical_classification ,Materials science ,Mechanical Engineering ,02 engineering and technology ,Polymer ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Viscoelasticity ,Condensed Matter::Soft Condensed Matter ,Colloid ,Creep ,chemistry ,Rheology ,Mechanics of Materials ,Chemical physics ,Colloidal gels ,LAOS ,Nonlinear rheology ,Steady shear ,Yield strain ,Yielding ,0103 physical sciences ,Volume fraction ,Particle ,General Materials Science ,Deformation (engineering) ,010306 general physics ,0210 nano-technology - Abstract
The depletion attraction, induced upon addition of a nonadsorbing polymer to a colloidal solution, can lead to gel formation at sufficiently high polymer concentrations, which corresponds to strong attractive interactions. We have investigated the nonlinear rheological response, in particular the yielding, of colloidal gels with an intermediate volume fraction and variable interparticle attraction. Two distinct yielding processes are observed in both oscillatory experiments, namely, dynamic strain sweeps and transient experiments, here step rate, creep, and recovery tests. The first yielding process occurs at strains similar to the range of the interparticle potential and is interpreted as the breaking of bonds, which destroys the particle network and leads to individual clusters. The process of bond breaking is successfully modeled as the escape of a particle from the potential well of its nearest neighbor. The second yield point occurs at larger strains and is related to the deformation and fragmentatio...
- Published
- 2011
36. Creep and flow of glasses:strain response linked to the spatial distribution of dynamical heterogeneities
- Author
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Jürgen Horbach, Stefan U. Egelhaaf, Pinaki Chaudhuri, Wilson C. K. Poon, Tatjana Sentjabrskaja, Michiel Hermes, and Marco Laurati
- Subjects
Multidisciplinary ,Materials science ,FOS: Physical sciences ,Mechanics ,Condensed Matter - Soft Condensed Matter ,SOFT ,Spatial distribution ,Article ,Amorphous solid ,NONLINEAR RHEOLOGY ,COLLOIDAL GLASSES ,Creep ,Flow (mathematics) ,Rheology ,Particle dynamics ,DEFORMATION ,SPHERE ,Soft Condensed Matter (cond-mat.soft) ,SHEAR ,Transient (oscillation) ,Strain response ,Rheology, Colloids, Glasses, Creep, Confocal Microscopy ,METALLIC GLASSES ,BEHAVIOR - Abstract
Mechanical properties are of central importance to materials sciences, in particular if they depend on external stimuli. Here we investigate the rheological response of amorphous solids, namely col- loidal glasses, to external forces. Using confocal microscopy and computer simulations, we establish a quantitative link between the macroscopic creep response and the microscopic single-particle dy- namics. We observe dynamical heterogeneities, namely regions of enhanced mobility, which remain localized in the creep regime, but grow for applied stresses leading to steady flow. These different behaviors are also reflected in the average particle dynamics, quantified by the mean squared dis- placement of the individual particles, and the fraction of active regions. Both microscopic quantities are found to be proportional to the macroscopic strain, despite the non-equilibrium and non-linear conditions during creep and the transient regime prior to steady flow., Comment: 10 pages, 6 figures
- Published
- 2015
37. Different mechanisms for dynamical arrest in largely asymmetric binary mixtures
- Author
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Andrew B. Schofield, J Hendricks, Marco Laurati, Stefan U. Egelhaaf, and R. F. Capellmann
- Subjects
Materials science ,Chemistry(all) ,media_common.quotation_subject ,Quantitative Biology::Tissues and Organs ,Binary number ,Physics and Astronomy(all) ,Asymmetry ,Quantitative Biology::Cell Behavior ,Condensed Matter::Soft Condensed Matter ,Colloid ,Chemical physics ,Volume fraction ,Colloids, Binary Mixtures, Glasses, Gels, Confocal Microscopy ,SPHERES ,Dynamical heterogeneity ,Large size ,media_common - Abstract
Using confocal microscopy we investigate binary colloidal mixtures with large size asymmetry, in particular the formation of dynamically arrested states of the large spheres. The volume fraction of the system is kept constant, and as the concentration of small spheres is increased we observe a series of transitions of the large spheres to different arrested states: an attractive glass, a gel, and an asymmetric glass. These states are distinguished by the degree of dynamical arrest and the amount of structural and dynamical heterogeneity. The transitions between two different arrested states occur through melting and the formation of a fluid state. While a space-spanning network of bonded particles is found in both arrested and fluid states, only arrested states are characterized by the presence of a space-spanning network of dynamically arrested particles.
- Published
- 2014
38. Poly(ethylene-alt-propylene)–poly(ethylene oxide) diblock copolymer micelles: a colloidal model system withtunable softness
- Author
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Jörg Stellbrink, Lutz Willner, Marco Laurati, Gernot Rother, Reidar Lund, and Dieter Richter
- Subjects
Materials science ,Ethylene oxide ,business.industry ,Context (language use) ,Hard spheres ,Condensed Matter Physics ,Small-angle neutron scattering ,Micelle ,chemistry.chemical_compound ,Optics ,Chemical engineering ,chemistry ,Phase (matter) ,Micellar solutions ,Copolymer ,General Materials Science ,business - Abstract
Kinetically frozen micelles formed by the amphiphilic diblock copolymer poly(ethylene-alt-propylene)?poly(ethylene oxide)?(PEP?PEO) are proposed as a new model system for soft colloids. In this context soft is used with a twofold meaning: the intraparticle softness, i.e.?the molecular architecture of an individual micelle, as well as the interparticle softness, i.e.?the effective potential for micellar interactions. Both contributions can be precisely adjusted from hard sphere-like to ultrasoft (star-like) by changing the diblock copolymer composition and/or interfacial tension, as shown by small angle neutron scattering in combination with contrast variation techniques. Depending on the degree of softness, PEP?PEO micellar solutions respond variably to the application of external shear fields. In particular, in the star-like regime solutions are already extremely sensitive at low shear rates. Therefore these micelles are an excellent starting point for a comprehensive study on the relation between softness and non-equilibrium phase behaviour in colloidal systems.
- Published
- 2004
39. Transient dynamics during stress overshoots in binary colloidal glasses
- Author
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Stefan U. Egelhaaf, Tatjana Sentjabrskaja, Ramón Castañeda-Priego, C. D. Estrada, Wilson C. K. Poon, Michiel Hermes, and Marco Laurati
- Subjects
Self-diffusion ,DISPERSIONS ,media_common.quotation_subject ,SUSPENSIONS ,Inverse ,HARD-SPHERE ,Asymmetry ,Colloid ,Rheology ,Mixing ratio ,PARTICLES ,media_common ,Chemistry ,MIXTURES ,General Chemistry ,Mechanics ,Condensed Matter Physics ,Shear rate ,Condensed Matter::Soft Condensed Matter ,Classical mechanics ,Shear (geology) ,SELF-DIFFUSION ,LIQUIDS ,SHEAR ,BEHAVIOR ,TRANSITION - Abstract
We investigate, using simultaneous rheology and confocal microscopy, the time-dependent stress response and transient single-particle dynamics following a step change in shear rate in binary colloidal glasses with large dynamical asymmetry and different mixing ratios. The transition from solid-like response to flow is characterised by a stress overshoot, whose magnitude is linked to transient superdiffusive dynamics as well as cage compression effects. These and the yield strain at which the overshoot occurs vary with the mixing ratio, and hence the prevailing caging mechanism. The yielding and stress storage are dominated by dynamics on different time and length scales, the short-time incage dynamics and the long-time structural relaxation respectively. These time scales and their relation to the characteristic time associated with the applied shear, namely the inverse shear rate, result in two different and distinct regimes of the shear rate dependencies of the yield strain and the magnitude of the stress overshoot.
- Published
- 2014
40. Discussion of 011405JOR by Colombo and Del Gado
- Author
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Ralph H. Colby, Roger T. Bonnecaze, Marco Laurati, D Gado, Dimitris Vlassopoulos, Alessio Zaccone, Colombo, and G. Petekidis
- Subjects
Materials science ,Mechanics of Materials ,Mechanical Engineering ,General Materials Science ,Condensed Matter Physics ,Humanities ,Rheology, Colloids, Gels - Published
- 2014
41. Residual Stresses in Glasses
- Author
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Kevin J. Mutch, George Petekidis, Miriam Siebenbürger, Thomas Voigtmann, Matthias Krüger, Matthias Ballauff, Nick Koumakis, Marco Laurati, Joseph M. Brader, Jochen Zausch, Stefan U. Egelhaaf, Matthias Fuchs, Jürgen Horbach, and Publica
- Subjects
Length scale ,Thermodynamic state ,FOS: Physical sciences ,General Physics and Astronomy ,02 engineering and technology ,Condensed Matter - Soft Condensed Matter ,glasses ,01 natural sciences ,Molecular dynamics ,Residual stress ,0103 physical sciences ,ddc:530 ,Ideal (ring theory) ,010306 general physics ,Physics ,Condensed Matter - Materials Science ,Condensed matter physics ,Materials Science (cond-mat.mtrl-sci) ,021001 nanoscience & nanotechnology ,Shear rate ,Condensed Matter::Soft Condensed Matter ,Flow (mathematics) ,residual stresses ,Soft Condensed Matter (cond-mat.soft) ,Relaxation (physics) ,rheology ,0210 nano-technology ,Rheology, Glasses, Residual Stresses, Mode Coupling Theory - Abstract
The history dependence of the glasses formed from flow-melted steady states by a sudden cessation of the shear rate $\dot\gamma$ is studied in colloidal suspensions, by molecular dynamics simulations, and mode-coupling theory. In an ideal glass, stresses relax only partially, leaving behind a finite persistent residual stress. For intermediate times, relaxation curves scale as a function of $\dot\gamma t$, even though no flow is present. The macroscopic stress evolution is connected to a length scale of residual liquefaction displayed by microscopic mean-squared displacements. The theory describes this history dependence of glasses sharing the same thermodynamic state variables, but differing static properties., Comment: submitted to Physical Review
- Published
- 2013
42. Yielding of binary colloidal glasses
- Author
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Eleftheria Babaliari, George Petekidis, Marco Laurati, Stefan U. Egelhaaf, Jan Hendricks, and Tatjana Sentjabrskaja
- Subjects
Materials science ,Random close pack ,FOS: Physical sciences ,02 engineering and technology ,General Chemistry ,Condensed Matter - Soft Condensed Matter ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Stress (mechanics) ,Condensed Matter::Soft Condensed Matter ,Colloid ,Rheology ,Volume (thermodynamics) ,Chemical physics ,0103 physical sciences ,Soft Condensed Matter (cond-mat.soft) ,SPHERES ,010306 general physics ,0210 nano-technology ,Cage ,Elastic modulus - Abstract
The rheological response, in particular the non-linear response, to oscillatory shear is experimentally investigated in colloidal glasses. The glasses are highly concentrated binary hard-sphere mixtures with relatively large size disparities. For a size ratio of 0.2, a strong reduction of the normalized elastic moduli, the yield strain and stress and, for some samples, even melting of the glass to a fluid is observed upon addition of the second species. This is attributed to the more efficient packing, as indicated by the shift of random close packing to larger total volume fractions. This leads to an increase in free volume which favours cage deformations and hence a loosening of the cage. Cage deformations are also favoured by the structural heterogeneity introduced by the second species. For a limited parameter range, we furthermore found indications of two-step yielding, as has been reported previously for attractive glasses. In samples containing spheres with more comparable sizes, namely a size ratio of 0.38, the cage seems less distorted and structural heterogeneities on larger length scales seem to become important. The limited structural changes are reflected in only a small reduction of the moduli, yield strain and stress., Comment: 9 pages, 8 figures, accepted in Soft Matter
- Published
- 2013
- Full Text
- View/download PDF
43. Glasses of dynamically asymmetric binary colloidal mixtures: Quiescent properties and dynamics under shear
- Author
-
M. Paul Lettinga, Marco Laurati, Donald Guu, Tatjana Sentjabrskaja, and Stefan U. Egelhaaf
- Subjects
Materials science ,media_common.quotation_subject ,FOS: Physical sciences ,Condensed Matter - Soft Condensed Matter ,Asymmetry ,Viscoelasticity ,Condensed Matter::Soft Condensed Matter ,Colloid ,Impurity ,Chemical physics ,Volume fraction ,Soft Condensed Matter (cond-mat.soft) ,SPHERES ,Anisotropy ,Elastic modulus ,binary mixtures ,dynamics ,glass ,structure ,viscoelasticity ,media_common - Abstract
We investigate mixing effects on the glass state of binary colloidal hard-sphere-like mixtures with large size asymmetry, at a constant volume fraction phi = 0.61. The structure, dynamics and viscoelastic response as a function of mixing ratio reflect a transition between caging by one or the other component. The strongest effect of mixing is observed in systems dominated by caging of the large component. The possibility to pack a large number of small spheres in the free volume left by the large ones induces a pronounced deformation of the cage of the large spheres, which become increasingly delocalised. This results in faster dynamics and a strong reduction of the elastic modulus. When the relative volume fraction of small spheres exceeds that of large spheres, the small particles start to form their own cages, slowing down the dynamics and increasing the elastic modulus of the system. The large spheres become the minority and act as an impurity in the ordering beyond the first neighbour shell, i.e. the cage, and do not directly affect the particle organisation on the cage level. In such a system, when shear at constant rate is applied, melting of the glass is observed due to facilitated out-of-cage diffusion which is associated with structural anisotropy induced by shear., 8 pages, 7 figures, Proceedings of the 4th International Symposium on Slow Dynamics in Complex Systems, Sendai, 2-7 December 2012
- Published
- 2013
44. Transient dynamics in dense colloidal suspensions under shear : shear rate dependence
- Author
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George Petekidis, Kevin J. Mutch, Jochen Zausch, John F. Brady, Christian P. Amann, Jürgen Horbach, Mattihus Fuchs, Marco Laurati, Nikolaos-Nektarios Koumakis, Andrew B. Schofield, and Stefan U. Egelhaaf
- Subjects
pacs:82.70.Dd Colloids ,62.10.+s Mechanical properties of liquids ,82.70.Kj Emulsions and suspensions ,61.20.Ja Computer simulation of liquid structure ,64.70.P- Glass transitions of specific systems ,Chemistry ,mechanical & thermal ,liquids and polymers ,structural [Condensed matter] ,Mechanics ,Condensed Matter Physics ,Simple shear ,Shear rate ,Shear modulus ,Physics::Fluid Dynamics ,Condensed Matter::Soft Condensed Matter ,Rheology ,Shear (geology) ,Critical resolved shear stress ,Chemical physics and physical chemistry ,Brownian dynamics ,General Materials Science ,ddc:530 ,Statistical physics ,Soft matter ,Shear flow - Abstract
A combination of confocal microscopy and rheology experiments, Brownian dynamics (BD) and molecular dynamics (MD) simulations and mode coupling theory (MCT) have been applied in order to investigate the effect of shear rate on the transient dynamics and stress-strain relations in supercooled and glassy systems under shear. Immediately after shear is switched on, the microscopic dynamics display super-diffusion and the macroscopic rheology a stress overshoot, which become more pronounced with increasing shear rate. MCT relates both to negative sections of the generalized shear modulus, which grow with increasing shear rate. When the inverse shear rate becomes much smaller than the structural relaxation time of the quiescent system, relaxation through Brownian motion becomes less important. In this regime, larger stresses are accumulated before the system yields and the transition from localization to flow occurs earlier and more abruptly.
- Published
- 2012
45. Yielding of hard-sphere glasses during start-up shear
- Author
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George Petekidis, John F. Brady, Marco Laurati, Stefan U. Egelhaaf, and Nick Koumakis
- Subjects
Materials science ,Rheometry ,Colloids, Glasses, Shear, Yielding, Simulations, Microscopy ,General Physics and Astronomy ,02 engineering and technology ,Mechanics ,021001 nanoscience & nanotechnology ,Breakup ,Thermal diffusivity ,Microstructure ,01 natural sciences ,Condensed Matter::Soft Condensed Matter ,Classical mechanics ,Shear (geology) ,Rheology ,0103 physical sciences ,Brownian dynamics ,010306 general physics ,0210 nano-technology ,Anisotropy - Abstract
Concentrated hard-sphere suspensions and glasses are investigated with rheometry, confocal microscopy, and Brownian dynamics simulations during start-up shear, providing a link between microstructure, dynamics, and rheology. The microstructural anisotropy is manifested in the extension axis where the maximum of the pair-distribution function exhibits a minimum at the stress overshoot. The interplay between Brownian relaxation and shear advection as well as the available free volume determine the structural anisotropy and the magnitude of the stress overshoot. Shear-induced cage deformation induces local constriction, reducing in-cage diffusion. Finally, a superdiffusive response at the steady state, with a minimum of the time-dependent effective diffusivity, reflects a continuous cage breakup and reformation.
- Published
- 2012
46. Dynamics of water absorbed in polyamides
- Author
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Jan Peter Embs, Angel Alegría, Paul Sotta, Juan Colmenero, Marco Laurati, Didier R. Long, Arantxa Arbe, Louise-Anne Fillot, Tobias Unruh, Gerald J. Schneider, Ministerio de Educación y Ciencia (España), European Commission, Donostia International Physics Center, Eusko Jaurlaritza, and Centre National de la Recherche Scientifique (France)
- Subjects
Arrhenius equation ,Materials science ,Polymers and Plastics ,Organic Chemistry ,Dielectric ,Neutron scattering ,Atmospheric temperature range ,Dielectric spectroscopy ,Amorphous solid ,Inorganic Chemistry ,Condensed Matter::Materials Science ,Crystallography ,Crystallinity ,symbols.namesake ,Chemical physics ,Materials Chemistry ,symbols ,Relaxation (physics) - Abstract
We investigate the dynamics of water absorbed in amorphous and semicrystalline aromatic polyamide copolymers. The combination of dielectric spectroscopy and quasi-elastic neutron scattering experiments allows us to characterize the water dynamics over a wide range of temperatures (dielectric spectroscopy) and at microscopic length scales (neutron scattering). The dielectric investigation evidences two relaxations associated with water motions: a fast process corresponding to motions of loosely bonded water molecules and a slower process corresponding to motions of amide-water complexes. While the slower process presents the characteristic Arrhenius temperature dependence of a secondary local relaxation over the whole temperature range, the fast process shows a crossover from Arrhenius to Vogel-Fulcher-Tamman (VFT) behavior at T ≈ 225 K, characteristic of confined water dynamics. The microscopic investigation by neutron scattering shows than in the VFT regime of the fast process the dynamics present a diffusive nature similar to bulk water. A large distribution of diffusion coefficients indicates possible differences in the connectivity of the hydrogen bond network. Diffusive heterogeneous dynamics can arise from a nonuniform distribution of water. The confinement effect of the polymer matrix is detected as a considerable reduction of the diffusion coefficient of water with respect to bulk. The presence of a crystalline phase results in a slowing down of both the fast and slow processes involving water motions. This could give a hint to the presence of a rigid amorphous phase in the semicrystalline material. 2012 American Chemical Society., We thank support by the DIPC, Rhodia, the European Commission NoE SoftComp, Contract NMP3- CT-2004-502235, the projects MAT2007-63681 (Spanish Ministerio de Educacion y Ciencia) and IT-436-07 (GV) (Basque Government). M.L. acknowledges support from CNRS.
- Published
- 2012
47. Structure, dynamics, and rheology of colloid-polymer mixtures: From liquids to gels
- Author
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Matthias Fuchs, Joseph M. Brader, George Petekidis, Stefan U. Egelhaaf, Marco Laurati, Nick Koumakis, Andrew B. Schofield, and Fred Cardinaux
- Subjects
chemistry.chemical_classification ,Materials science ,Chromatography ,General Physics and Astronomy ,Polymer ,Light scattering ,Viscoelasticity ,Gel, Colloid, Structure, Dynamics, Light Scattering ,Condensed Matter::Soft Condensed Matter ,Colloid ,Rheology ,chemistry ,Dynamic light scattering ,Chemical physics ,Volume fraction ,Static light scattering ,ddc:530 ,Physical and Theoretical Chemistry - Abstract
We investigate the structural, dynamical, and viscoelastic properties of colloid-polymer mixtures at intermediate colloid volume fraction and varying polymer concentrations, thereby tuning the attractive interactions. Within the examined range of polymer concentrations, the samples varied from fluids to gels. In the liquid phase, an increasing correlation length of the density fluctuations when approaching the gelation boundary was observed by static light scattering and microscopy, indicating clustering and formation of space-spanning networks. Simultaneously, the correlation function determined by dynamic light scattering decays completely, indicating the absence of dynamical arrest. Clustering and formation of transient networks when approaching the gelation boundary is supported by significant changes in the viscoelastic properties of the samples. Upon increasing the polymer concentration beyond the gelation boundary, the rheological properties changed qualitatively again, now they are consistent with the formation of colloidal gels. Our experimental results, namely, the location of the gelation boundary as well as the elastic (storage) and viscous (loss) moduli, are compared to different theoretical models. These include consideration of the escape time as well as predictions for the viscoelastic moduli based on scaling relations and mode coupling theories.
- Published
- 2009
48. From equilibrium to steady state: The transient dynamics of colloidal liquids under shear
- Author
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Thomas Voigtmann, Matthias Fuchs, Joseph M. Brader, Stefan U. Egelhaaf, Jürgen Horbach, Marco Laurati, Jochen Zausch, and Publica
- Subjects
Materials science ,Gaussian ,FOS: Physical sciences ,Condensed Matter - Soft Condensed Matter ,confocal microscopy ,Molecular dynamics ,symbols.namesake ,Colloid ,ddc:530 ,General Materials Science ,Colloids, Glasses, Shear, Dynamics, Transient ,Magnetosphere particle motion ,glass forming liquids under shear ,mode coupling serie ,Disordered Systems and Neural Networks (cond-mat.dis-nn) ,Mechanics ,Condensed Matter - Disordered Systems and Neural Networks ,Condensed Matter Physics ,Start up ,molecular dynamics ,Condensed Matter::Soft Condensed Matter ,Shear (geology) ,Homogeneous ,Mode coupling ,symbols ,Soft Condensed Matter (cond-mat.soft) - Abstract
We investigate stresses and particle motion during the start up of flow in a colloidal dispersion close to arrest into a glassy state. A combination of molecular dynamics simulation, mode coupling theory and confocal microscopy experiment is used to investigate the origins of the widely observed stress overshoot and (previously not reported) super-diffusive motion in the transient dynamics. A link between the macro-rheological stress versus strain curves and the microscopic particle motion is established. Negative correlations in the transient auto-correlation function of the potential stresses are found responsible for both phenomena, and arise even for homogeneous flows and almost Gaussian particle displacements., Comment: 24 pages, 14 figures, J. Phys.: Condens. Matter, in press
- Published
- 2008
49. Small-angle neutron scattering of mixed ionic perfluoropolyether micellar solutions
- Author
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José A. Teixeira, Piero Baglioni, Cecilia M. C. Gambi, R. Pieri, R. Giordano, Marco Laurati, and A. Chittofrati
- Subjects
chemistry.chemical_classification ,Analytical chemistry ,Neutron scattering ,Micelle ,Small-angle neutron scattering ,PHASE-BEHAVIOR ,Surfaces, Coatings and Films ,chemistry ,Pulmonary surfactant ,Micellar solutions ,DODECYL-SULFATE MICELLES ,Materials Chemistry ,Micellar cubic ,Organic chemistry ,WATER ,Microemulsion ,Physical and Theoretical Chemistry ,Counterion ,SURFACTANTS ,CARBOXYLIC SALTS - Abstract
Aqueous mixed micellar solutions of perfluoropolyether carboxylic salts with ammonium counterions have been studied by small-angle neutron scattering. Two surfactants differing in the tail length were mixed in proportions n2/n3 = 60/40 w/w, where n2 and n3 are the surfactants with two and three perfluoroisopropoxy units in the tail, respectively. The tails are chlorine-terminated. The mixed micellar solutions, in the concentration range 0.1-0.2 M and thermal interval 20-40 degrees C, show structural characteristics of the interfacial shell that are very similar to ammonium n2 micellar solutions previously investigated; thus, the physics of the interfacial region is dominated by the polar head and counterion. The shape and dimensions of the micelles are influenced by the presence of the n3 surfactant, whose chain length in the micelle is 2 A longer than that of the n2 surfactant. The n3 surfactant favors the ellipsoidal shape in the concentration range 0.1-0.2 M with a 1/2 ionization degree of n2 micelles. The very low surface charge of the mixed micelles is attributed to the increase in hydrophobic interactions between the surfactant tails, due to the longer n3 surfactant molecules in micelles. The closer packing of the tails decreases the micellar curvature and the repulsions between the polar heads, by surface charge neutralization of counterions migrating from the Gouy-Chapman diffuse layer, leading to micellar growth in ellipsoids with greater axial ratios.
- Published
- 2007
50. Asymmetric poly(ethylene-alt-propylene)-poly(ethylene oxide) micelles: A system with starlike morphology and interactions
- Author
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Emanuela Zaccarelli, Reidar Lund, Lutz Willner, Jörg Stellbrink, Dieter Richter, and Marco Laurati
- Subjects
Materials science ,Micelles, Copolymers, Star Polymers, Neutron Scattering ,Thermodynamics ,SMALL-ANGLE SCATTERING ,Neutron scattering ,Atomic packing factor ,Small-angle neutron scattering ,POLYMERIC MICELLES ,law.invention ,Amorphous solid ,Condensed Matter::Soft Condensed Matter ,Crystal ,NEUTRON-SCATTERING ,law ,Phase (matter) ,BLOCK-COPOLYMER MICELLES ,POLY(ETHYLENE OXIDE) ,Crystallization ,Phase diagram - Abstract
We report on an experimental study of single particle properties and interactions of poly(ethylene-alt-propylene)-poly(ethylene oxide) (PEP-PEO) starlike micelles. The starlike regime is achieved by an extremely asymmetric block ratio (1:20) and the number of arms (functionality) is changed by varying the composition of the solvent (the interfacial tension). Small angle neutron scattering (SANS) data in the dilute regime can be modeled by assuming a constant density profile in the micellar core (compact core) and a starlike density profile in the corona (starlike shell). The starlike morphology of the corona is confirmed by a direct comparison with SANS measurements of dilute poly butadiene star solutions. Comparison of structure factors obtained by SANS measurements in the concentrated regime shows in addition that the interactions in the two systems are equivalent. Micellar structure factors at several packing fractions can be modeled by using the ultrasoft potential recently proposed for star polymers [Likos et al., Phys. Rev. Lett. 80, 4450 (1998)]. The experimental phase diagram of PEP-PEO micelles is quantitatively compared to theoretical expectations, finding good agreement for the location of the liquid-solid boundary and excellent agreement for the critical packing fraction where the liquid-to-bcc crystal transition takes place for $fl70$. The functionality, i.e., the coronal density, strongly influences the nature of the solid phase: for $fl70$ the system crystallizes into a bcc phase, high $fg70$ formation of amorphous arrested states prevents crystallization.
- Published
- 2007
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