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Nanopillar-textured surfaces are of growing interest because of their ability to kill bacteria through physical damage without relying on antimicrobial chemicals. Although research on antibacterial nanopillars has progressed significantly in recent years, the effect of nanopillar hydrophobicity on bactericidal activity remains elusive. In this study, we investigated the mechano-bactericidal efficacy of etched silicon nanopillars against more...

The streaming potentials of hyaluronic acid (HA) hydrogel films are measured and theoretically interpreted by systematically varying the HA concentration and the streaming electrolyte pH and ionic strength. While Donnan potentials are expected to vanish with sufficient added salt, apparent ζ-potentials from the Helmholtz-Smoluchowski interpretation remain of the order -20 mV. To theoretically interpret these data, we derived an electrokinetic model (valid in the Debye-Hückel regime) that accounts for ionic and hydrodynamic permeability of the gels. The films could then be ascribed an effective acid dissociation constant p more...

Hydrogels are promising supports for nanoemulsion drops since the skeleton provides a physical barrier to coalescence and possibly a thermodynamic barrier to Ostwald ripening. How these factors pla... more...

Dynamic mobility spectra of sodium dodecyl sulphate (SDS)-stabilized hexadecane nanodrops in aqueous NaCl electrolytes are measured using the electrokinetic-sonic amplitude, and interpreted using a... more...

As a first step toward understanding the electroacoustic response of polyelectrolyte hydrogel nanocomposites, we systematically vary the linear charge density of acrylic acid-co-acrylamide polymer ... more...

We examine the swelling and dissolution of hydrogels cross-linked by a combination of physical and chemical junctions, focusing on the technologically important DNA- and “bis”-cross-linked polyacry... more...

We examine networks of complementary-DNA-strand cross-linked polyacrylamide, with and without covalent N,N′-methylene(bis)acrylamide cross-linking, using rheological time–temperature superposition ... more...

Nanostructured, inorganic microspheres have many industrial applications, including catalysis, electronics, and particularly drug delivery, with several advantages over their organic counterparts. However, many current production methods require high energy input, use of harmful chemicals, and extensive processing. Here, the self-assembly of calcium pyrophosphate into nanofibre microspheres is reported. This process takes place at ambient temperature, with no energy input, and only salt water as a by-product. The formation of these materials is examined, as is the formation of nanotubes when the system is agitated, from initial precipitate to crystallisation. A mechanism of formation is proposed, whereby the nanofibre intermediates are formed as the system moves from kinetically favoured spheres to thermodynamically stable plates, with a corresponding increase in aspect ratio. The functionality of the nanofibre microspheres as targeted enteric drug delivery vehicles is then demonstrated in vitro and in vivo, showing that the microspheres can pass through the stomach while protecting the activity of a model protein, then release their payload in intestinal conditions. more...

An electrokinetic model for a surfactant-stabilized nano-drop under oscillatory forcing is solved. This generalizes a model for which an analytical solution was recently proposed for large, highly charged drops. Calculations of the dynamic electrophoretic mobility and the accompanying electrostatic polarization for a single drop provide a theoretical foundation for interpreting electrokinetic sonic amplitude and complex-conductivity spectra for dilute surfactant-stabilized oil-in-water emulsions and bubbly liquids. The model is distinguished from earlier models by accounting for the internal fluid and interfacial dynamics at finite frequencies ( ). This dynamics accounts for the electro-migration, diffusion and advection of surfactant ions on the interface, and exchange of these ions with the immediately adjacent electrolyte. Surface gradients induce Marangoni stresses, which couple to the electrical and hydrodynamic stresses, modulating the magnitude and phase of the drop velocity and electrostatic polarization induced by the electric field. Of particular interest, for sodium dodecyl sulphate stabilized oil-in-water drops, is how the high surface-charge density manifests in a breakdown of the Smoluchowski-slip approximation, even for drops with very thin diffuse layers. More generally, the model furnishes dynamic mobilities for drops with arbitrary size and charge, thus permitting appropriate averaging for polydisperse systems. Such calculations may help to resolve long-standing challenges and controversy with regards to the surface-charge density of nano-drops and their macro-scale counterparts, and may pave the way to quantitative interpretations of more complex dynamic interfacial rheology and exchange kinetics, e.g. for Pickering emulsions. more...

A theoretical analysis of the dynamic electrophoretic mobility of surfactant-stabilized nano-drops is undertaken. Whereas the theory for rigid spherical nanoparticles is well developed, its application to nano-drops is questionable due to fluid mobility of the interface and of the surfactant molecules adsorbed there. At zero frequency, small drops with surface impurities are well known to behave as rigid spheres due to concentration-gradient-induced Marangoni stresses. However, at the megahertz frequencies of electroacoustic (and other spectral-based) diagnostics, the interfacial concentration gradients are dynamic, coupling electromigration, advection and diffusion fluxes. This study addresses a parameter space that is relevant to anionic-surfactant-stabilized oil–water emulsions, using sodium-dodecylsulfate-stabilized hexadecane as a specific example. The drop size is several hundred nanometres, much larger than the diffuse-layer thickness, thus motivating thin-double-layer approximations. The theory demonstrates that fluid mobility and fluctuating Marangoni stresses can have a profound influence on the magnitude and phase of the dynamic mobility. We show that the drop interface transits from a rigid/immobile one at low frequency to a fluid one at high frequency. The model unifies electrokinetics and equilibrium interfacial thermodynamics. Therefore, with knowledge of how the interfacial tension varies with electrolyte composition (oil, surfactant and added salt concentrations), the particle radius might be adopted as the primary fitting parameter (rather than the customary -potential) from an experimental measure of the dynamic mobility. This theory is general enough that it might be applied to aerosols and bubbly dispersions (at sufficiently high frequencies). more...

Chemically cross-linked polyacrylamide (PA) hydrogels have been studied extensively, but gels with a very low cross-linker ratio have received relatively little attention. In this paper, the rheology of bis-cross-linked PA hydrogels is examined with acrylamide concentrations c a approximately 10% w/v and cross-linker to acrylamide ratios c bis / c a in the range 0.2– 0.9 mmol mol − 1. Linear viscoelastic responses were measured during and following gelation. The effective cross-link formation rate increases with the cross-linker ratio, with a plateau modulus that is weakly quadratic in the cross-linker ratio. The gels exhibit distinctly different features from their counterparts in the literature with higher cross-linker ratios. Plateau regions in the dynamic moduli spectra are less pronounced, and the loss tangents are several orders of magnitude higher than for gels with a higher cross-linker ratio but a comparable plateau modulus. Time-temperature superposition of viscoelastic spectra furnishes a disentanglement activation energy that decreases with increasing temperature, from ≈ 25 to 6 k B T. Time-concentration superposition for samples prepared above the percolation threshold furnishes a critical relaxation exponent Δ ≈ 0.38, which is much smaller than previously reported for the universal sol-gel transition. These distinct features can be attributed to the predominance of entanglements. We show that a Wiechert model with a power-law distribution of relaxation times faithfully reproduces dynamic moduli spectra from the creep compliance. Together, the results provide a foundation with which to interpret acoustic and electroacoustic rheological responses (at MHz frequencies) and the linear viscoelasticity of DNA-cross-linked gels (to be reported elsewhere).Chemically cross-linked polyacrylamide (PA) hydrogels have been studied extensively, but gels with a very low cross-linker ratio have received relatively little attention. In this paper, the rheology of bis-cross-linked PA hydrogels is examined with acrylamide concentrations c a approximately 10% w/v and cross-linker to acrylamide ratios c bis / c a in the range 0.2– 0.9 mmol mol − 1. Linear viscoelastic responses were measured during and following gelation. The effective cross-link formation rate increases with the cross-linker ratio, with a plateau modulus that is weakly quadratic in the cross-linker ratio. The gels exhibit distinctly different features from their counterparts in the literature with higher cross-linker ratios. Plateau regions in the dynamic moduli spectra are less pronounced, and the loss tangents are several orders of magnitude higher than for gels with a higher cross-linker ratio but a comparable plateau modulus. Time-temperature superposition of viscoelastic spectr... more...

Many systems of biophysical and technological interest consist of multiple interpenetrating chains in a confined volume, i.e., a confined polymer solution. Using nanofluidic approaches developed originally for the study of single chains in confined geometries, we develop an assay to create confined polymer solutions on-chip and then probe the solution response to applied compressive forcing. In our approach, multiple chains are introduced into a nanoslit via hydrodynamic flow and are then concentrated against a barrier that is permeable only to solvent. For sufficiently high concentration, the compressed solution profile can be described by a mean-field polymer model based on Doi’s two-fluid approach, with the chain free energy described by a Ginzburg-type free energy functional. This theory furnishes a partial differential equation based description of the concentration profile in terms of a nonlinear Schrodinger-type equation, providing a general theoretical framework for modeling confined polymer solut... more...

The effective diffusivity of two-dimensional, anisotropic porous materials with surface diffusion is studied. The continuum model of Albaalbaki and Hill (2012) , which couples diffusion in the bulk and surface domains via interfacial exchange fluxes, is implemented to couple the phases. Using a cell model, a new analytical solution is developed for aligned fibres with elliptic cross-sections and arbitrary orientation with respect to the mean gradient. The anisotropic boundary-value-problem is solved using an isotropic approximation to furnish concentration distributions in the three phases. Therefore, the model is more accurate near the isotropic limit and at lower inclusion volume fractions. When surface diffusion is significant, the present anisotropic model reproduces the isotropic model of Albaalbaki and Hill (2012) for unit aspect ratio and a variety of physical parameters. For a sphere with negligible surface flux, the model agrees with Maxwell’s theory, and reproduces the model of Akbari et al. (2013) with various aspect ratios. To test the model for several parameters and other aspect ratios, direct numerical computations of the effective diffusivity, using spatially periodic unit cells, are undertaken, and a comparison with experimental data is presented. This model serves as a two-dimensional solution for the effective diffusivity of dilute anisotropic structures with surface diffusion. more...

Nanoparticles (NPs) have been demonstrated in recent years to simultaneously enhance polymeric nanocomposite membrane selectivity and permeability according to the polymer/NP/penetrant. With a repulsive polymer-NP interaction, permselectivity is enhanced by NP loading, which is thought to adjust the permeability by locally disturbing polymer conformation. In this paper, we examine another mechanism for controlling nanocomposite permeability. We focus on the interfacial sorption capacity and mobility of water-vapour through nanocomposites comprising spherical silica NPs embedded in cross-linked polyacrylamide. Equilibrium and dynamic moisture sorption data are interpreted with a quantitative theoretical model that addresses equilibrium and dynamic partitioning of the tracer between interfacial and continuous phases. The results demonstrate significant and systematic changes in non-classical diffusion behaviour. Our interpretation indicates that polyacryamide adsorption on silica NPs varies the number of available surface adsorption sites, thus controlling adsorption capacity and interfacial mobility. Such effects can therefore be tuned by varying the specific NP surface area, which increases with NP loading and decreases with NP size. These insights will hopefully improve rational design strategies for nanocomposite membranes. more...

Nanoparticle–hydrogel interactions are important in many applications. In drug delivery, for example, these control the release rate and may prevent nanoparticle (NP) migration from a targeted site. In this paper, electroacoustic spectroscopy is used to study the NP–hydrogel interaction, focusing on the influence of polymer adsorption and hydrogel composition. Electroacoustic spectroscopy is a powerful noninvasive tool to complement microrheological characterization. We study the interaction of polyacrylamide (PA) with laponite and silica NPs as model systems with strong and weak attraction, respectively. Stronger adsorption of PA on laponite compared to silica imparts distinctly different rheological properties to PA solutions and decreases the laponite mobility significantly more than silica when embedded in PA hydrogels. However, the mobilities of both NPs exhibit qualitatively similar variations with acrylamide and chemical cross-linker concentrations, as indicated by viscoelastic and electrokinetic c... more...

Pore-resolved computations are undertaken, within a continuum model framework, to explore surface diffusion as a selective mechanism for gas separations using membranes of randomly overlapping parallel cylindrical pores or fibers. Orders of magnitude of the model parameters are established using an intrinsic gas diffusivity that is self-consistent with the Knudsen diffusivity obtained from Monte Carlo simulations reported in the literature. The relative contributions of surface and gas diffusion to the overall permeation show that the surface-diffusion flux increases with the specific surface area, whereas the gas flux increases with porosity. Thus, gas diffusion that is perpendicular to pores and fibers can be hindered by the increasing tortuosity while simultaneously promoting permeation via surface diffusion. The selectivity of pore structures with fibrous networks is examined for the dehumidification of air, natural gas, and carbon dioxide. Selectivities, defined as the ratio of the effective diffusio... more...

The sol-gel transition of silica-polyacrylamide hydrogel nanocomposites is studied when approaching the gel point by independently varying the nanoparticle (NP) and chemical crosslinker concentrations. Time-concentration-superposition (TCS) principles are applied to ascertain the effect of NPs on dynamics at the gel point. Doping dilute and concentrated polyacrylamide (PA) solutions with silica NPs indicates that these particles do not crosslink PA; neither do they form percolating networks. Nevertheless, silica NPs interact with PA to influence the storage and loss moduli to a much greater degree than expected for passive fillers. Interestingly, when silica NPs are embedded in very weakly chemically crosslinked PA (with a bis crosslinker concentration that would otherwise not form PA hydrogels), they significantly enhance the effective degree of crosslinking, forming viscoelastic solids. From TCS analysis of dynamics at the gel point, the bond probability is found to scale with the NP volume fraction as p ∼ ϕ 1.5 , whereas chemical crosslinking furnishes a bond probability that is linear in the crosslinker concentration. more...

Three-dimensional computations of coupled gas and surface diffusion in periodic arrays of solid spheres and spherical cavities are undertaken to assess the role of pore size, void fraction, and pore geometry on permeability. With nano-dimensioned pores, gas-phase diffusion is hindered by the small mean-free path, so the overall permeability hinges on surface diffusion. We prescribe the intrinsic gas-phase diffusivity to be self-consistent with the Knudsen diffusivity obtained from molecular-kinetic simulations reported in the literature. The continuum model of Albaalbaki and Hill then couples the gas and surface phases via an equilibrium adsorption isotherm and kinetic-exchange parameters. Solving these equations in complex pore-scale geometries provides new insights into the coupling. To interpret the results, we decompose the net flux into (i) gas and surface contributions that can be calculated without knowledge of the concentration perturbations to equilibrium, but which depend on the pore geometry, and (ii) counterparts that must be evaluated from integrals of the concentration perturbations to equilibrium. These measures are directly related to the familiar, but less precise, gas and surface tortuosity parameters, which are often used to interpret experiments. The results highlight (i) discontinuous variations in the fluxes when passing through void and solid percolation thresholds, (ii) how the surface flux depends on the specific surface area, and (iii) the pore sizes at which surface transport compares with void transport. more...

This paper examines the nanoparticle (NP) influence on energy storage and dissipation in hydrogel nanocomposites (HNCs). To obtain fundamental insights into mechanical enhancement, a model system involving the in situ free-radical polymerization of acrylamide with bis-acrylamide (bis) and silica NPs is adopted. The loss tangents of the unmodified polymer networks span three orders of magnitude, and the weak attraction between silica and poly(acrylamide) (PA)—as compared to composites with a stronger NP-polymer interaction—makes these HNCs particularly sensitive to systematic variations in (i) NP size and concentration and (ii) monomer and crosslinker concentrations. From the dynamic shear moduli during polymerization, and their spectra at steady-state, silica NPs in PA behave as multi-functional, physical crosslinking centers that increase the storage modulus, particularly in very weakly bis-crosslinked PA (in which NP aggregates are proposed to form elastically effective clusters). The loss modulus for HNCs reflects adsorption/desorption and friction at the NP surfaces, varying with the NP, monomer and crosslinker concentrations. Silica NPs were also found to slow the polymerization and crosslinking of acrylamide according to the specific NP surface area (set by the NP size and concentration), suggesting that silica NPs reduce the free-radical concentration. more...

Polyacrylamide (PA) hydrogels have been studied extensively, but fundamental aspects of their gelation kinetics, percolation dynamics, and viscoelasticity are still not well understood. This paper focuses on the rheology of PA hydrogels having unusually low monomer concentrations (ca ≈ 3 w% equivalent to 0.42 mol l−1). These furnish loss tangents that span 4 orders of magnitude when varying the crosslinker concentration. An optimum crosslinker concentration (cbis/ca ≈ 2.5 mol. % equivalent to 5.3 w%) is identified, below which the storage modulus G′ increases almost linearly, and the loss modulus G″ acquires a local maximum. Above the optimum crosslinker concentration, G′ and G″ both plateau, accompanied by a notable decrease in the maximum strain (increase in brittleness) before breaking. The dynamic shear moduli reveal universal dynamics at the gel point, as indicated by (i) scaling exponents (y = 3.1 ± 0.1, z = 2.1 ± 0.1 and Δ = 0.70 ± 0.02) that are consistent with the de Gennes [“On a relation betwee... more...

A general flux equation for multicomponent diffusion in oxide melts is presented. An explicit method was developed to calculate the gradients of single-ion activities from those of oxides with the constraints of local equilibrium and electroneutrality. This resolves ambiguity in quantifying the thermochemical driving force for ionic diffusion. A model equation for multicomponent ionic diffusion was derived within the framework of non-equilibrium thermodynamics by de Groot and Mazur. The proposed model takes empirically measurable quantities as input variables, so the diffusion calculations are consistent with thermochemical data, as furnished by the CALPHAD (CALculation of PHAse Diagrams) method, as well as ionic mobility measurements. Although the model is derived for oxides, it can be applied to diffusion in other concentrated liquid electrolytes, such as chloride and fluoride melts. Formulas for multicomponent ionic diffusion in various reference frames are presented with respect to mole fraction. more...

Foam formation during vesiculation of hydrous magmatic melts at 1 atm was studied in situ by synchrotron X-ray tomographic microscopy at the TOMCAT beamline of the Swiss Light Source (Villigen, Switzerland). Four different compositions were studied; basaltic, andesitic, trachyandesitic and dacitic hydrous glasses were synthesized at high pressures as starting materials and then laser heated on the beamline. The porosity, bubble number density, size distributions of bubbles and pore throats, as well as the tortuosity and connectivity of bubbles in the foams, were measured in three dimensions based on tomographic reconstructions of sample volumes. The reconstructed volumes were also used in lattice-Boltzmann simulations to determine viscous permeabilities of the samples. Connectivity of bubbles by pore throats varied from ~100 to 105 mm-3, and for each sample correlated positively with porosity and permeability. Although permeability increased with porosity, the relationship is complex; consideration of the results of this and previous studies of the viscous permeabilities of aphyric and crystal-poor magmatic samples demonstrated that at similar porosities the permeability could vary by many orders of magnitude, even in similar composition samples. More than 90 % of these permeabilities are bounded by two empirical power laws, neither of which identifies a percolation threshold. Comparison of the permeability relationships from this study with previous models (Degruyter et al. 2010; Burgisser et al. 2017) relating porosity, characteristic porethroat diameters and tortuosity demonstrated good agreement. However, modifying the Burgisser et al. (2017) model by using the maximum measured pore-throat diameter, instead of the average diameter, as the characteristic diameter produced a model that reproduced the lattice-Boltzmann permeabilities to within 1 order of magnitude. Measured correlations between the average bubble diameter and the maximum pore-throat diameter as well as between porosity and tortuosity in our experiments produced relationships that allow application of the modified Burgisser et al. model to predict permeability based only upon the average bubble diameter and porosity. The experimental results are consistent with previous studies suggesting that increasing bubble growth rates result in decreasing permeability of equivalent porosity foams. The effect of growth rate on permeability is hypothesized to substantially contribute to the multiple orders-of-magnitude variations in the permeabilities of natural magmatic samples at similar porosities. more...

Nanoparticle zeta-potentials are relatively easy to measure, and have consistently been proposed in guidance documents as a particle property that must be included for complete nanoparticle characterization. There is also an increasing interest in integrating data collected on nanomaterial properties and behavior measured in different systems (e.g. in vitro assays, surface water, soil) to identify the properties controlling nanomaterial fate and effects, to be able to integrate and reuse datasets beyond their original intent, and ultimately to predict behaviors of new nanomaterials based on their measured properties (i.e. read across), including zeta-potential. Several confounding factors pose difficulty in taking, integrating and interpreting this measurement consistently. Zeta-potential is a modeled quantity determined from measurements of the electrophoretic mobility in a suspension, and its value depends on the nanomaterial properties, the solution conditions, and the theoretical model applied. The ability to use zeta-potential as an explanatory variable for measured behaviors in different systems (or potentially to predict specific behaviors) therefore requires robust reporting with relevant meta-data for the measurement conditions and the model used to convert mobility measurements to zeta-potentials. However, there is currently no such standardization for reporting in the nanoEHS literature. The objective of this tutorial review is to familiarize the nanoEHS research community with the zeta-potential concept and the factors that influence its calculated value and interpretation, including the effects of adsorbed macromolecules. We also provide practical guidance on the precision of measurement, interpretation of zeta-potential as an explanatory variable for processes of interest (e.g. toxicity, environmental fate), and provide advice for addressing common challenges associated with making meaningful zeta-potential measurements using commercial instruments. Finally, we provide specific guidance on the parameters that need to be reported with zeta-potential measurements to maximize interpretability and to support scientific synthesis across data sets. more...

Gel-electrophoresis has been demonstrated in recent decades to successfully sort a great variety of nanoparticles according to their size, charge, surface chemistry, and corona architecture. However, quantitative theoretical interpetations have been limited by the number and complexity of factors that influence particle migration. Theoretical models have been fragmented and incomplete with respect to their counterparts for free-solution electrophoresis. This paper unifies electrokinetic models that address complex nanoparticle corona architectures, corona and gel charge regulation (e.g., by the local pH), multi-component electrolytes, and non-linear electrostatics and relaxation effects. By comprehensively addressing the electrokinetic aspects of the more general gel-electrophoresis problem, in which short-ranged steric interactions are significant, a stage is set to better focus on the physicochemical and steric factors. In this manner, it is envisioned that noparticle gel-electrophoresis may eventually be advanced from a nanoparticle-characterization tool to one that explicitly probes the short-ranged interactions of nanoparticles with soft networks, such as synthetic gels and biological tissues. In this paper, calculations are undertaken that identify a generalized Hückel limit for nanoparticles in low-conductivity gels, and a new Smoluchowski limit for polyelectrolyte-coated particles in high-conductivity gels that is independent of the gel permeability. Also of fundamental interest is a finite, albeit small, electrophoretic mobility for uncharged particles in charged gels. Electrophoretic mobilities and drag coefficients (with electroviscous effects) for nanoparticles bearing non-uniform coronas show that relaxation effects are typically weak for the small nanoparticles (radius ≈3-10 nm) to which gel-electrophoresis has customarily been applied, but are profound for the larger nanoparticles (radius ≳ 40 nm in low conductivity gels) to which passivated gel-electrophoresis experiments have recently been applied. To demonstrate its practical application, the model is applied to (pH charge regulating) carboxylated polystyrene nanospheres in low-density passivated agarose gels (weak steric effects). This furnishes a new theoretical interpretation of literature data for which a finite diffuse-layer-thickness, pH-charge regulation, high charge, and relaxation effects dominate over the steric influences. more...

Wood-fibre conformation affects paper properties in various paper-product categories, such as packaging, printing, and absorbents. Qualitative investigations suggest that capillary forces play a crucial role in determining the fibre conformation upon drying. To quantify this process, we theoretically and experimentally investigate deformation of a circular tube under capillary pressure. Fibre pit holes, which impose a significant capillary pressure while drying, are modelled as circular holes in a tube. The calculations are undertaken by coupling the analytical solution for buckling a circular tube to numerical solutions of the Young–Laplace equation. This elasto-capillary model elucidates the influences of wetting angle, tube-wall flexibility, and hole size on the tube deformation. In experiments, flexible silicon-rubber tubes with a hole in the wall are filled with liquid, and the internal pressure is measured while withdrawing this fluid, thus mimicking evaporation during the drying of wood fibres. The results prove that capillarity can collapse the fibre lumen, either partially or completely, depending principally on the fibre-wall flexibility. more...

Electro-dewatering (ED) is a novel technology to reduce the overall costs of residual biosolids processing, transport, and disposal. In this study, we investigated Joule heating and pH as parameters controlling the dewaterability limit, dewatering rate, and energy efficiency. Temperature-controlled electrodes revealed that Joule heating enhances water removal by increasing evaporation and electro-osmotic flow. High temperatures increased the dewatering rate, but had little impact on the dewaterability limit and energy efficiency. Analysis of horizontal layers after 15-min ED suggests electro-osmotic flow reversal, as evidenced by a shifting of the point of minimum moisture content from the anode toward the cathode. This flow reversal was also confirmed by the pH at the anode being below the isoelectric point, as ascertained by pH titration. The important role of pH on ED was further studied by adding acid/base solutions to biosolids prior to ED. An acidic pH reduced the biosolids charge while simultaneously increasing the dewatering efficiency. Thus, process optimization depends on trade-offs between speed and efficiency, according to physicochemical properties of the biosolids microstructure. more...

Free volume and polymer chain architecture play important roles in controlling the glass transition temperature $$T_g$$ of polymer nanocomposites. Various changes in $$T_g$$ with respect to nanoparticle (NP) loading have been reported, depending, in part, on whether there are attractive or repulsive interactions between the polymer and NPs. However, even with no enthalpic interaction, there are ostensible changes in $$T_g$$ that must be attributed to topological factors, such as chain stiffness and nanoparticle size. Here we adopt a macroscopic granular model to help understand frustrated dynamics in glassy polymer nanocomposites. Mixtures of granular chains with spherical inclusions were prepared with prescribed sphere size, chain length, and mixture composition. We measured the time to reach a close–packed, jammed state when these composites were subjected to controlled mechanical shaking. The compaction dynamics reveal that spherical inclusions profoundly influence the chain relaxation dynamics. In the long-chain limit, increasing the NP loading furnishes a minimum in the chain relaxation time, which may be loosely associated with an intermediate minimum in $$T_g$$ with respect to nanoparticle loading for polymer nanocomposites. This minimum occurs for spheres having different sizes, but only at concentrations where the characteristic sphere separation is comparable to the chain loop size. This observation may explain the variety of contrasting trends that have been found in the literature for the dependence of $$T_g$$ on nanoparticle loading in polymeric nanocomposites. more...

We study the stability and breakup of liquid bridges with a free contact line on surfaces with contact-angle hysteresis (CAH) under zero-gravity conditions. Non-ideal surfaces exhibit CAH because of surface imperfections, by which the constraints on three-phase contact lines are influenced. Given that interfacial instabilities are constraint-sensitive, understanding how CAH affects the stability and breakup of liquid bridges is crucial for predicting the drop size in contact-drop dispensing. Unlike ideal surfaces on which contact lines are always free irrespective of surface wettability, contact lines may undergo transitions from pinned to free and vice versa during drop deposition on non-ideal surfaces. Here, we experimentally and theoretically examine how stability and breakup are affected by CAH, highlighting cases where stability is lost during a transition from a pinned–pinned (more constrained) to pinned-free (less constrained) interface—rather than a critical state. This provides a practical means of expediting or delaying stability loss. We also demonstrate how the dynamic contact angle can control the contact-line radius following stability loss. more...

Dynamics of colloidal particles adhering to soft, deformable substrates, such as tissues, biofilms, and hydrogels play a key role in many biological and biomimetic processes. These processes, including, but not limited to colloid-based delivery, stitching, and sorting, involve microspheres exploring the vicinity of soft, sticky materials in which the colloidal dynamics are affected by the fluid environment (e.g., viscous coupling), inter-molecular interactions between the colloids and substrates (e.g., Derjaguin-Landau-Verwey-Overbeek (DLVO) theory), and the viscoelastic properties of contact region. To better understand colloidal dynamics at soft interfaces, an optical tweezers back-focal-plane interferometry apparatus was developed to register the transverse Brownian motion of a silica microsphere in the vicinity of polyacrylamide (PA) hydrogel films. The time-dependent mean-squared displacements are well described by a single exponential relaxation, furnishing measures of the transverse interfacial diffusion coefficient and binding stiffness. Substrates with different elasticities were prepared by changing the PA crosslinking density, and the inter-molecular interactions were adjusted by coating the microspheres with fluid membranes. Stiffer PA hydrogels (with bulk Young's moduli ≈1-10 kPa) immobilize the microspheres more firmly (lower diffusion coefficient and position variance), and coating the particles with zwitterionic lipid bilayers (DOPC) completely eliminates adhesion, possibly by repulsive dispersion forces. Remarkably, embedding polyethylene glycol-grafted lipid bilayers (DSPE-PEG2k-Amine) in the zwitterionic fluid membranes produces stronger adhesion, possibly because of polymer-hydrogel attraction and entanglement. This study provides new insights to guide the design of nanoparticles and substrates with tunable adhesion, leading to smarter delivery, sorting, and screening of micro- and nano-systems. more...

Literature interpretations of the electrophoretic mobility of spherical polyelectrolytes are revisited using the capillary-electrophoresis data of Duval et al. (2006) for the extracellular polysaccharide succinoglycan as an example. Subtle changes in the polyelectrolyte mobility have recently been attributed to new electrokinetic theories that feature multi-component electrolytes, charge regulation, and the so-called polarization and relaxation phenomena. However, these calculations exhibit several unusual trends that have yet to be explained, and so the conclusions drawn from them are controversial. Here, independent computations strengthen conclusions drawn from the original model of Duval et al., i.e., the discrepancies between experiments and all the presently available electrokinetic theories reflect changes in the conformation of succinoglycan arising from changes in the electrolyte pH and ionic strength. more...

Anchoring poly(ethylene glycol) (PEG) to inorganic nanoparticles (NPs) permits control over NP properties for a variety of technological applications. However, the core-shell structure tremendously complicates the interpretation of the ubiquitous ζ-potential, as furnished by electrophoretic light-scattering, capillary electrophoresis or gel electrophoresis. To advance the ζ-potential-and the more fundamental electrophoretic mobility-as a quantitative diagnostic for PEGylated NPs, we synthesized and characterized Au NPs bearing terminally anchored 5 kDa PEG ligands with univalent carboxymethyl end groups. Using the electrophoretic mobilities, acquired over a wide range of ionic strengths, we developed a theoretical model for the distributions of polymer segments, charge, electrostatic potential, and osmotic pressure that envelop the core: knowledge that will help to improve the performance of soft NPs in fundamental research and technological applications. more...

Liquid droplets bridging filaments are ubiquitous in nature and technology. Although the liquid-surface shape and the capillary force and torque have been studied extensively, the effect of filament flexibility is poorly understood. Here, we show that elastic deformation (at large values of the elasto-capillary number) can significantly affect the liquid-surface shape and capillary force. The equilibrium state of parallel filaments is calculated using analytical approximations and numerical solutions for the fluid interface. The results compare well, and the numerical solution is then applied to crossing filaments. In the investigated range of parameters, the capillary force increases rapidly when the filaments touch. The force decreases continuously when decreasing the liquid volume for parallel hydrophilic filaments but produces a maximum for crossed filaments. The liquid volume at the maximum force is reported when changing the filament flexibility, crossing angle, and contact angle. These results may be beneficial in applications where the strength and structure of wet fibrous materials are important, such as in paper formation and when welding flexible components. more...

We examine the equilibrium and stability of an elastocapillary system to model drying-induced structural failures. The model comprises a circular elastic membrane with a hole at the center that is deformed by the capillary pressure of simply connected and doubly connected menisci. Using variational and spectral methods, stability is related to the slope of equilibrium branches in the liquid content versus pressure diagram for the constrained and unconstrained problems. The second-variation spectra are separately determined for the membrane and meniscus, showing that the membrane out-of-plane spectrum and the in-plane spectrum at large elastocapillary numbers are both positive, so that only meniscus perturbations can cause instability. At small elastocapillary numbers, the in-plane spectrum has a negative eigenvalue, inducing wrinkling instabilities in thin membranes. In contrast, the smallest eigenvalue of the meniscus spectrum always changes sign at a pressure turning point where stability exchange occurs in the unconstrained problem. We also examine configurations in which the meniscus and membrane are individually stable, while the elastocapillary system as a whole is not; this emphasizes the connection between stability and the coupling of elastic and capillary forces. more...

Gel electrophoresis of spherical nanoparticles (NPs) is studied using an electrokinetic model that couples the ion conservation equations to the Poisson and fluid momentum equations, thus including the so-called polarization and relaxation processes. This model is therefore the charged gel electrophoresis analogue of the well-known O’Brien and White solution of the standard electrokinetic model for free-solution electrophoresis. Results are provided for the small NPs (size around 10 nm) to which gel electrophoresis is relevant, because particles must be small enough to permeate the gel: these include the particle drag coefficient (or Brownian diffusivity), which is subject to hydrodynamic screening and electroviscous effects, and the electrophoretic mobility, which is subject to nonlinear electrostatic and charge polarization influences. Also addressed are the influences of charge-regulating gels and the accompanying particle-induced immobile charge-density perturbations. Ion-concentration perturbations attenuate the electrophoretic mobility and enhance the drag coefficient according to the particle charge and the mobility of the most abundant counterion. However, dynamic regulation of the hydrogel charge—termed the secondary immobile charge-density perturbation—has a negligible influence on the particle mobility, and may therefore be neglected for most practical purposes. more...

An elastocapillary model for drying-induced collapse is proposed. We consider a circular elastic membrane with a hole at the centre that is deformed by the capillary pressure of simply and doubly connected menisci. The membrane overlays a cylindrical cavity with rigid walls, trapping a prescribed volume of water. This geometry may be suitable for studying structural failures and stiction in micro-electromechanical systems during wet etching, where capillary surfaces experience catastrophic transitions. The dry state is determined using the dihedral-angle and volume-turning-point stability criteria. Open and collapsed conformations are predicted from the scaled hole radius, cavity aspect ratio, meniscus contact angle with the membrane and cavity walls, and an elastocapillary number measuring the membrane stretching rigidity relative to the water surface tension. For a given scaled hole radius and cavity aspect ratio, there is a critical elastocapillary number above which the system does not collapse upon drying. The critical elastocapillary number is weakly influenced by the contact angle over a wide range of the scaled hole radius, thus indicating a limitation of surface hydrophobization for controlling the dry-state conformation. The model is applied to the drying of wood fibres above the fibre saturation point, determining the conditions leading to collapse. more...