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Light-driven actuation of synthetic polymers is an emerging field of interest because it offers simple remote addressing without complicated hydraulic, electric, or magnetic systems. Reviews on this area predominantly emphasize on the development of mechanical motions like bending, twisting, folding, etc. However, the scientific and fundamental aspects of these materials are critical in order to expand applications and industrial relevance. Polymer actuators driven by light, not only comprise soft actuators (large deformations at low stress) but also include stiff actuators (high actuation stress at low strain). Synthetic polymeric materials with photo-responsive additives together with underlying mechanisms, processing parameters, and final properties are required to broaden the scope of the field. In particular, parameters like actuation stress, actuation strain, and work capacity have been given limited attention in the past and are discussed extensively. This work gives a comprehensive critical review on all light-driven synthetic polymer actuators, their actuating mechanisms, and materials. A holistic perspective together with an insight into future prospects can lead academia and industry toward future innovations and applications of these exciting functional materials.

A study of phase transitions of photochromic p,p’-n-alkylazobenzenes compounds was carried out by means of adiabatic scanning calorimetry (ASC) and photopyroelectric calorimetry (PPE) techniques, yielding accurately the temperature dependence of the enthalpy and of the specific heat capacity across the observed phase transitions, and the corresponding enthalpy of transition values. In addition, polarization microscopy imaging of the sample texture was performed together with the PPE calorimetric evaluations to establish the nature of the involved phases. By performing PPE calorimetric evaluations during UV irradiation of the samples, the influence of the photoinduced trans to cis isomerization of the nAB molecules on the occurrence of the phase transitions was assessed.

Nematic liquid crystals have been known for more than a century, but it was not until the 60s-70s that, with the development of room temperature nematics, they became widely used in applications. Polar nematic phases have been long-time predicted, but have only been experimentally realized recently. Synthesis of materials with nematic polar ordering at room temperature is certainly challenging and requires a deep understanding of its formation mechanisms, presently lacking. Here, we compare two materials of similar chemical structure and demonstrate that just a subtle change in the molecular structure enables denser packing of the molecules when they exhibit polar order, which shows that reduction of excluded volume is in the origin of the polar nematic phase. Additionally, we propose that molecular dynamics simulations are potent tools for molecular design in order to predict, identify and design materials showing the polar nematic phase and its precursor nematic phases., 33 pages, 21 of main manuscript and 12 of Supporting Information, 7 main figures

Growing tissue and bacterial colonies are active matter systems where cell divisions and cellular motion generate active stress. Although they operate in the non-equilibrium regime, these biological systems can form large-scale ordered structures. How mechanical instabilities drive the dynamics of active matter systems and form ordered structures are not well understood. Here, we use chaining Bacillus subtilis, also known as a biofilm, to study the relation between mechanical instabilities and nematic ordering. We find that bacterial biofilms have intrinsic length scales above which a series of mechanical instabilities occur. Localized stress and friction drive buckling and edge instabilities which further create nematically aligned structures and topological defects. We also observe that topological defects control stress distribution and initiate the formation of sporulation sites by creating three-dimensional structures. In this study we propose an alternative active matter platform to study the essential roles of mechanics in growing biological tissue., Nature Communications, 10, ISSN:2041-1723

This thesis aims to demonstrate how the achromatic nature and design flexibility of liquid-crystal optics can be used to improve high-contrast imaging instruments to facilitate detailed exoplanet characterization.Chapter 2 discusses the design, performance, and future development of the liquid-crystal vector-apodizing phase plate (vAPP) coronagraph, five of which have been installed in different instruments on current generation telescopes since 2016. In chapter 3 we use the achromatic nature of the vAPP in combination with the LBT/ALES integral field spectrograph to obtain the first ever thermal infrared spectrum of the inner three HR 8799 planets. In Chapter 4 and 8 we show that by combining multiple grating patterns to reduce the influence of polarization leakage, we can improve the performance of liquid-crystal coronagraphs. In Chapter 5 and 6 we enhance sparse aperture masking, capable of detecting companions beyond the diffraction limit, by using liquid-crystal phase masks to enable low-resolution spectroscopy and improve throughput. In Chapter 7 we demonstrate that a liquid-crystal Zernike wavefront sensor can accurately and efficiently measure phase and amplitude aberrations simultaneously, facilitating extreme contrasts. Together, the concepts presented in this thesis can be used to improve high-contrast imaging instruments of both ground-based and space-based observatories.

Properties of soft materials are influenced by their anisotropic structuring under nonequilibrium fields. Although anisotropic structure-property relationships have been extensively explored theoretically, comparison to experiments requires determination of the microstructural orientation probability distribution function (OPDF) of microstructural elements. Small angle scattering (SAS) measurements encode information about the OPDF, but tools to navigate this connection are incomplete. Here, we develop and validate an explicit framework to link arbitrary OPDFs to SAS measurements. Specifically, we propose, validate, and apply a method, maximum a posteriori scattering inference (MAPSI), whereby the OPDF may be obtained from SAS measurements using a Bayesian estimation method. Using this method, we obtain estimates of the full 3D OPDF for two model semidilute fd-virus (rodlike) dispersions at concentrations that are approximately equal to and twice the overlap concentration. From the OPDF, we calculate its second and fourth moments and compare these to predictions for a dilute suspension of rigid rods and to a recent theory for semidilute suspensions. Finally, we use both the theoretical and measured moments to calculate the stress, both for dilute and semidilute suspensions. These predictions are not only compared to each other, but also to measured values of the shear stress, and point to new insights into the behavior of suspensions of highly elongated particles in the transition between dilute and semidilute behavior. We also use this new framework to provide perspective on the connection between scalar parameterizations of scattering and the OPDF that have frequently been used in the past. The new tools developed in this work provide an unprecedented path toward experimental validation of dynamical theories of rodlike colloids and polymers, and for measurement of nonequilibrium structures and stresses of other complex fluids and soft materials with SAS.

Many ecosystems develop strikingly regular spatial patterns because of small-scale interactions between organisms, a process generally referred to as spatial self-organization. Self-organized spatial patterns are important determinants of the functioning of ecosystems, promoting the growth and survival of the involved organisms, and affecting the capacity of the organisms to cope with changing environmental conditions. The predominant explanation for self-organized pattern formation is spatial heterogeneity in establishment, growth and mortality, resulting from the self-organization processes. A number of recent studies, however, have revealed that movement of organisms can be an important driving process creating extensive spatial patterning in many ecosystems. Here, we review studies that detail movement-based pattern formation in contrasting ecological settings. Our review highlights that a common principle, where movement of organisms is density-dependent, explains observed spatial regular patterns in all of these studies. This principle, well known to physics as the Cahn-Hilliard principle of phase separation, has so-far remained unrecognized as a general mechanism for self-organized complexity in ecology. Using the examples presented in this paper, we explain how this movement principle can be discerned in ecological settings, and clarify how to test this mechanism experimentally. Our study highlights that animal movement, both in isolation and in unison with other processes, is an important mechanism for regular pattern formation in ecosystems. (C) 2016 Elsevier B.V. All rights reserved.

Reversible noncovalent but sequence-dependent attachment of DNA to gels is shown to allow programmable mobility processing of DNA populations. The covalent attachment of DNA oligomers to polyacrylamide gels using acrydite-modified oligonucleotides has enabled sequence-specific mobility assays for DNA in gel electrophoresis: sequences binding to the immobilized DNA are delayed in their migration. Such a system has been used for example to construct complex DNA filters facilitating DNA computations. However, these gels are formed irreversibly and the choice of immobilized sequences is made once off during fabrication. In this work, we demonstrate the reversible self-assembly of gels combined with amphiphilic DNA molecules, which exhibit hydrophobic hydrocarbon chains attached to the nucleobase. This amphiphilic DNA, which we term lipid-DNA, is synthesized in advance and is blended into a block copolymer gel to induce sequence-dependent DNA retention during electrophoresis. Furthermore, we demonstrate and characterize the programmable mobility shift of matching DNA in such reversible gels both in thin films and microchannels using microelectrode arrays. Such sequence selective separation may be employed to select nucleic acid sequences of similar length from a mixture via local electronics, a basic functionality that can be employed in novel electronic chemical cell designs and other DNA information-processing systems.

Chiral azobenzenes can be used as photoswitchable dopants to control supramolecular helices in liquid crystals. However, the lack of thermal stability of the cis-isomer precludes envisioning the generation of long-lived supramolecular helices with light. Here, this study demonstrates thermally stable and axially chiral azobenzene switches that can be used as chiral dopants to create supramolecular helices from (achiral) nematic liquid crystals. Their trans-to-cis photoisomerization leads to a variation of helical twisting power that reaches up to 60%, and the helical superstructure that is engineered with light displays a relaxation time that reaches tens of hours. These results demonstrate that combining ortho-fluorination with axial chirality in molecular photoswitches constitutes an efficient strategy to promote long-lived helical states. Further, this approach shows potential to design supramolecular machines that are controlled by light entirely.

International audience; We study the structure and liquid-crystalline phase behaviour of a model of confined non-convex circular soft-repulsive nanorings in a planar slit geometry using molecular-dynamics simulation. The separation distance between the structureless parallel soft-repulsive walls is made large enough to allow for the formation of a distinct bulk phase in the central region of the box which is in coexistence with the adsorbed fluid thus allowing the analysis of single-wall effects. As the density of the particles is increased, the fluid adsorbs (wets) onto the planar surfaces leading to the formation of well-defined smectic-A layers with a spacing proportional to the diameter of the rings. An analysis of the nematic order parameter at distances perpendicular to the surface reveals that the particles in each layer exhibit anti-nematic behaviour and planar (edge-on) anchoring relative to the short symmetry axis of the rings. This behaviour is in stark contrast to the behaviour observed in convex disc-like particles that have the tendency to form nematic (discotic) structures with homeotropic (face-on) anchoring. The smectic phases formed by nanorings in the bulk and under confinement are characterised by the formation of low-density layered liquid-crystalline states with large voids, referred to here as lacuna smectic phases. In contrast to what is typically found for confined liquid-crystalline systems involving convex particles, no apparent biaxiality is found for nanorings in planar confinement. We argue that formation of the low-density lacuna smectic layers with planar anchoring is a consequence of the non-convex shape of the circular rings that allow for interpenetration between the particles as observed for nanorings under bulk conditions [C. Avendaño, G. Jackson, E.A. Müller and F.A. Escobedo, Proc. Natl. Acad. Sci. U.S.A. 113, 9699 (2016); H.H. Wensink and C. Avendaño, Phys. Rev. E 94, 062704 (2016)]. ARTICLE HISTORY

The field of dynamic functional molecular systems has progressed enormously over the past few decades. By coupling the mechanical properties of molecular switches and motors to chemical and biological processes, exceptional control of function has been attained. Overcrowded alkene-based light-driven molecular motors are very attractive in this respect owing to their unique multistate photochemically and thermally induced switching processes and their helical chirality inversion in each switching step. However, extending our control over properties from the molecular scale to larger length scales is still a fundamental challenge. In this Perspective, we discuss recent developments that address this challenge, ranging from the application of these motors in catalysis and synthetic materials to the control of biological properties. We may now be positioned at the dawn of a new era in which artificial molecular motors are able to perform programmed tasks and dynamic functions akin to the biological machines that are found in daily life.

Graphene exhibits a range of exceptional physical properties and holds great promise for development of novel bulk materials for widespread applications. Properly engineering of assembled structures of graphene at multiple length scales is essential to realize its full potential in bulk forms. In this Concept article, we highlight the unique colloidal and gelation behavior of a commonly used precursor for graphene, graphene oxide (GO), and discuss how the colloidal chemistry of GO and reduced GO can enable new scalable and cost-effective approaches to construct graphene-based soft superstructures with excellent properties.

We report control over the phase behavior of CdS nanorods via the solvent and acidity. CdS nanorods were synthesized using alkane phosphonic acid ligands, which were replaced after synthesis by a series of aromatic ligands. Change of ligand enabled us to cast films from different solvents. By replacing toluene with ethanol or water the rod-rod interactions dominate over rod-substrate interactions, thereby favoring simple hexagonal ordering (2D). When dispersed in water, a net electrostatic charge on the nanorods could be induced by deprotonating the ligands at high pH. This net charge favors 2D nematic ordering over homeotropic ordering of the nanorods on a substrate. A calculation of the van der Waals and electrostatic interactions is presented that explains the observed influence of solvent and pH.

Light localization via reorientation in nematic liquid crystals supports multi-component optical spatial solitons, i.e., vector nematicons. By launching three optical beams of different wavelengths and the same input polarization in a bias-free planar cell, we demonstrate a three-color vector nematicon which is self-trapped thanks to its incoherent nature. Full Text: PDF References G. I. Stegeman and M. Segev, "Optical Spatial Solitons and Their Interactions: Universality and Diversity", Science 286 (5444), 1518 (1999) CrossRef W. Krolikowski and O. Bang, "Solitons in nonlocal nonlinear media: Exact solutions", Phys. Rev. E 63, 016610 (2000) CrossRef D. Suter and T. Blasberg, "Stabilization of transverse solitary waves by a nonlocal response of the nonlinear medium", Phys. Rev. A 48, 4583 (1993) CrossRef G. Assanto and M. Peccianti, "Spatial solitons in nematic liquid crystals", IEEE J. Quantum Electron. 39 (1), 13 (2003). CrossRef G. Assanto and M. Karpierz, "Nematicons: self-localised beams in nematic liquid crystals", Liq. Cryst. 36 (10), 1161 (2009) CrossRef M. Peccianti and G. Assanto, "Nematicons", Phys. Rep. 516, 147 (2012). CrossRef M. Peccianti and G. Assanto, "Incoherent spatial solitary waves in nematic liquid crystals", Opt. Lett. 26 (22), 1791 (2001) CrossRef M. Peccianti and G. Assanto, "Nematic liquid crystals: A suitable medium for self-confinement of coherent and incoherent light", Phys. Rev. E Rap. Commun. 65, 035603 (2002) CrossRef G. Assanto, M. Peccianti, C. Umeton, A. De Luca and I. C. Khoo, "Coherent and Incoherent Spatial Solitons in Bulk Nematic Liquid Crystals", Mol. Cryst. Liq. Cryst. 375, 617 (2002) CrossRef A. Alberucci, M. Peccianti, G. Assanto, A. Dyadyusha and M. Kaczmarek, "Two-Color Vector Solitons In Nonlocal Media", Phys. Rev. Lett. 97, 153903 (2006) CrossRef G. Assanto, N. F. Smyth and A. L. Worthy, "Two-color, nonlocal vector solitary waves with angular momentum in nematic liquid crystals", Phys. Rev. A 78 (1), 013832 (2008) CrossRef G. Assanto, K. Garcia-Reimbert, A. A. Minzoni, N. F. Smyth and A. Worthy, "Lagrange solution for three wavelength solitary wave clusters in nematic liquid crystals", Physica D 240, 1213 (2011) CrossRef G. Assanto, A. A. Minzoni and N. F. Smyth, "Vortex confinement and bending with nonlocal solitons", Opt. Lett. 39 (3), 509 (2014) CrossRef G. Assanto, A. A. Minzoni and N. F. Smyth, "Deflection of nematicon-vortex vector solitons in liquid crystals", Phys. Rev. A 89, 013827 (2014) CrossRef G. Assanto and N. F. Smyth, "Soliton Aided Propagation and Routing of Vortex Beams in Nonlocal Media", J. Las. Opt. Photon. 1, 105 (2014) CrossRef Y. V. Izdebskaya, G. Assanto and W. Krolikowski, "Observation of stable-vector vortex solitons", Opt. Lett. 40 (17), 4182 (2015) CrossRef Y. V. Izdebskaya, W. Krolikowski, N. F. Smyth and G. Assanto, "Vortex stabilization by means of spatial solitons in nonlocal media", J. Opt. 18 (5), 054006 (2016) CrossRef J. F. Henninot, J. Blach and M. Warenghem, "Experimental study of the nonlocality of spatial optical solitons excited in nematic liquid crystal", J. Opt. A 9, 20 (2007) CrossRef Y. V. Izdebskaya, V. G. Shvedov, A. S. Desyatnikov, W. Z. Krolikowski, M. Belic, G. Assanto and Y. S. Kivshar, "Counterpropagating nematicons in bias-free liquid crystals", Opt. Express 18 (4), 3258 (2010) CrossRef N. Karimi, A. Alberucci, M. Virkki, M. Kauranen and G. Assanto, "Phase-front curvature effects on nematicon generation", J. Opt. Soc. Am. B 5 (33), 903 (2016) CrossRef P. G. de Gennes and J. Prost, The Physics of Liquid Crystals, Oxford Science Publications (Clarendon Press, 2nd edition, 1993) I. C. Khoo, Liquid Crystals: Physical Properties and Nonlinear Optical Phenomena (Wiley, New York, 1995) A. Piccardi, M. Trotta, M. Kwasny, A. Alberucci, R. Asquini, M. Karpierz, A. d'Alessandro and G. Assanto, "Trends and trade-offs in nematicon propagation", Appl. Phys. B 104 (4), 805 (2011) CrossRef M. Kwasny, U. A. Laudyn, F. A. Sala, A. Alberucci, M. A. Karpierz and G. Assanto, "Self-guided beams in low-birefringence nematic liquid crystals", Phys. Rev. A 86 (1), 01382 (2012) CrossRef M. Peccianti, A. Fratalocchi and G. Assanto, "Transverse dynamics of nematicons", Opt. Express 12 (26), 6524 (2004) CrossRef C. Conti, M. Peccianti and G. Assanto, "Observation of Optical Spatial Solitons in a Highly Nonlocal Medium", Phys. Rev. Lett. 92 (11), 113902 (2004) CrossRef A. Alberucci, C.-P. Jisha and G. Assanto, "Breather solitons in highly nonlocal media", J. Opt. 18, 125501 (2016) CrossRef

We discuss the propagation of an electromagnetic field in an inhomogeneously anisotropic material where the optic axis is rotated in the transverse plane but is invariant along the propagation direction. In such a configuration, the evolution of an electromagnetic wavepacket is governed by the Pancharatnam-Berry phase (PBP), responsible for the appearance of an effective photonic potential. In a recent paper [A. Alberucci et al., "Electromagnetic confinement via spin-orbit interaction in anisotropic dielectrics", ACS Photonics \textbf{3}, 2249 (2016)] we demonstrated that the effective potential supports transverse confinement. Here we find the profile of the quasi-modes and show that the photonic potential arises from the Kapitza effect of light. The theoretical results are confirmed by numerical simulations, accounting for the medium birefringence. Finally, we analyze in detail a configuration able to support non-leaky guided modes., 14 pages, 13 figures

International audience; The synthesis of metal oxide nanoparticles (NPs) from organometallic precursor is a powerful approach to access nanomaterials of controllable size, shape, and surface; parameters of prime importance for their applications. Our recent efforts in this field is presented.

In this thesis, we study entropy-driven phase transitions in suspensions of colloidal particles. Colloids are small particles, with typical sizes ranging from the nanometer to the micron, dispersed in a medium that is composed of much smaller particles (atoms or molecules). Because of this size difference, colloids experience Brownian motion that allows them to move around the medium, exploring the microscopic configurations available, and possibly arrange themselves in ordered structures. This process is called self-assembly. For example, colloids self-assemble in fluids (disordered arrangements of particles), liquid-crystals (partially ordered), and crystals (fully ordered). The stability and the transitions between these phases, i.e., the phase behaviour, depend both on the interactions between the colloids and on thermodynamic parameters like temperature or density. Here, we focus on systems composed of hard colloidal particles, that are particles that do not have any interactions except for the fact that cannot overlap with each other. By using computer simulations and theory, we show that the particle shape is enough for the formation of several thermodynamic phases. In other words, it is possible to obtain order in complete absence of any attraction in the system, just by increasing the density of the system. These transitions are (fully) entropy-driven since no change in energy is associated to the phase transition but only a change in entropy, that is a quantity related to the number of possible microscopic particle arrangements. In chapter 2 we show that when (tens of) thousands of hard spheres are compressed, while being confined in a spherical cavity, they do not self-assemble into an FCC crystal, that is the equilibrium bulk crystal structure, but rather they form stable icosahedral clusters. In chapter 3, we study hard spherocylinders forming liquid crystals. In chapter 4, we study liquid crystals formed by binary mixtures of colloidal rods and spheres, focusing in particular on the binary smectic phase, that consists in alternating layers of rods and spheres. In chapters 2, 3, and 4 our theoretical and simulation results are compared with experiments performed in our group. In chapter 5, we consider hard rod-like particles with a polyhedral shape and we identify the conditions to form prolate, oblate and biaxial nematic phases. In chapters 6, 7 and 8, we consider colloidal particles with a chiral shape forming chiral liquid-crystal phases. In particular, in chapter 6 we develop a theory to predict the equilibrium cholesteric pitch as a function of thermodynamic state and microscopic details. Applying the theory to hard helices, we observe both right- and left-handed cholesteric phases that depend on a subtle combination of particle geometry and system density. In chapter 7, we introduce particles with a twisted polyhedral shape and obtain, for the first time, a stable fully-entropy-driven cholesteric phase by computer simulations. Our results unveil how the competition between particle biaxiality and chirality is transmitted at a higher level into the nematic phases and new theoretical challenges on the self-assembly of chiral particles are addressed in chapter 8.

In this thesis, we study entropy-driven phase transitions in suspensions of colloidal particles. Colloids are small particles, with typical sizes ranging from the nanometer to the micron, dispersed in a medium that is composed of much smaller particles (atoms or molecules). Because of this size difference, colloids experience Brownian motion that allows them to move around the medium, exploring the microscopic configurations available, and possibly arrange themselves in ordered structures. This process is called self-assembly. For example, colloids self-assemble in fluids (disordered arrangements of particles), liquid-crystals (partially ordered), and crystals (fully ordered). The stability and the transitions between these phases, i.e., the phase behaviour, depend both on the interactions between the colloids and on thermodynamic parameters like temperature or density. Here, we focus on systems composed of hard colloidal particles, that are particles that do not have any interactions except for the fact that cannot overlap with each other. By using computer simulations and theory, we show that the particle shape is enough for the formation of several thermodynamic phases. In other words, it is possible to obtain order in complete absence of any attraction in the system, just by increasing the density of the system. These transitions are (fully) entropy-driven since no change in energy is associated to the phase transition but only a change in entropy, that is a quantity related to the number of possible microscopic particle arrangements. In chapter 2 we show that when (tens of) thousands of hard spheres are compressed, while being confined in a spherical cavity, they do not self-assemble into an FCC crystal, that is the equilibrium bulk crystal structure, but rather they form stable icosahedral clusters. In chapter 3, we study hard spherocylinders forming liquid crystals. In chapter 4, we study liquid crystals formed by binary mixtures of colloidal rods and spheres, focusing in particular on the binary smectic phase, that consists in alternating layers of rods and spheres. In chapters 2, 3, and 4 our theoretical and simulation results are compared with experiments performed in our group. In chapter 5, we consider hard rod-like particles with a polyhedral shape and we identify the conditions to form prolate, oblate and biaxial nematic phases. In chapters 6, 7 and 8, we consider colloidal particles with a chiral shape forming chiral liquid-crystal phases. In particular, in chapter 6 we develop a theory to predict the equilibrium cholesteric pitch as a function of thermodynamic state and microscopic details. Applying the theory to hard helices, we observe both right- and left-handed cholesteric phases that depend on a subtle combination of particle geometry and system density. In chapter 7, we introduce particles with a twisted polyhedral shape and obtain, for the first time, a stable fully-entropy-driven cholesteric phase by computer simulations. Our results unveil how the competition between particle biaxiality and chirality is transmitted at a higher level into the nematic phases and new theoretical challenges on the self-assembly of chiral particles are addressed in chapter 8.

Two vinyl-terminated bent core-shaped liquid crystalline molecules that exhibit thermotropic antiferroelectric SmCPA phases have been covalently attached onto a hydrogen-terminated silicon(111) surface. The surface attachment was achieved via a mild procedure from a mesitylene solution, using visible tight at room temperature. AFM measurements indicate that a smooth monolayer has been formed. The thickness of the monolayer was evaluated with ellipsometry and X-ray reflectivity. Although the molecules differ in length by four carbon atoms, the thickness of the resulting monolayers was the same. The measured thicknesses correspond quite well with the smectic layer thickness in the bulk liquid crystalline material, suggesting a similar self-organization within the monolayer. From attenuated total reflectance infrared (ATR-IR), which clearly shows the C-H and C=O vibrations, a tilt angle of the mesogens is deduced that also corresponds well with the tilt angle in the liquid crystalline state. X-ray photoelectron spectroscopy (XPS) measurements confirm the high quality of the monolayers, with only marginal silicon oxide formation. The elemental composition and amounts of different O and C atoms deduced from the high-resolution XPS correspond very well with the calculated compositions.

Monte Carlo simulations of lattice spin models are employed to investigate the effects of an external field on the creation and evolution of inversion walls in nematic films with conical boundary conditions. The simulations, based on the well known Lebwohl-Lasher potential, allow to produce simulated polarized microscopy images which are followed during their evolution when the field is applied.

We have investigated the structural and dynamic properties of Xe dissolved in the ionic liquid crystal (ILC) phase of 1-hexadecyl-3-methylimidazolium nitrate using classical molecular dynamics (MD) simulations. Xe is found to be preferentially dissolved within the hydrophobic environment of the alkyl chains rather than in the ionic layers of the smectic phase. The structural parameters and the estimated local diffusion coefficients concerning the short-time motion of Xe are used to parametrize a theoretical model based on the Smoluchowski equation for the macroscopic dynamics across the smectic layers, a feature which cannot be directly obtained from the relatively short MD simulations. This protocol represents an efficient combination of computational and theoretical tools to obtain information on slow processes concerning the permeability and diffusivity of the xenon in smectic ILCs.

Diacetylenes have been the subject of current research because of their interesting optoelectronic properties. Herein, we report that substituted diphenylbutadiynes exhibit locally excited (LE) and excimer emissions in water and multiple emissions from the LE, excimer, and intramolecular charge transfer (ICT) states in acetonitrile-water solvent systems. The LE, excimer, and ICT emissions are clearly distinguishable for a diphenylbutadiynyl derivative with push (-NMe2)-pull (-CN) substituents and those are closely overlapped for non-push-pull analogues. In neat acetonitrile, the excimer emission disappears and the LE and ICT emissions predominate. In the case of the push (-NMe2)-pull (-CN) diphenylbutadiyne, the intensity of the ICT emission increases with increasing the fluorophore concentration. This suggests that the ICT emission accompanies with intermolecular CT emission which is of exciplex type. As the LE and exciplex emissions of the push-pull diphenylbutadiyne together cover the visible region (400-700 nm) in acetonitrile, a control of the fluorophore concentration makes the relative intensities of the LE and exciplex emissions such that pure white light emission is achieved. The white light emission is not observed in those diphenylbutadiynyl analogues in which the peripheral substituents of the phenyl rings do not possess strong push-pull character.

Recent advances in synthetic methods and analysis techniques provide a basis for the construction and characterization of organized arrays of molecular switches and motors on surfaces. Among them, molecular systems that can be controlled by light are particularly promising because of their ease of addressability, fast response times and the compatibility of light with a wide range of condensed phases. The aim of this contribution is to highlight selected recent advances in building functional monolayers of light-activated molecules. Special focus is given to monolayers of molecules whose collective switching properties have been harnessed to produce macroscopic effects. The design, structure, and function of monolayers composed of bistable photochromic switches, which can control chirality, wettability, conductivity and self-assembly are described. A recent report on the successful demonstration of light-driven rotary motors functioning while grafted on gold surfaces will also be discussed, followed by a brief conclusion. (C) 2007 Elsevier Ltd. All rights reserved.

Reversible noncovalent but sequence-dependent attachment of DNA to gels is shown to allow programmable mobility processing of DNA populations. The covalent attachment of DNA oligomers to polyacrylamide gels using acrydite-modified oligonucleotides has enabled sequence-specific mobility assays for DNA in gel electrophoresis: sequences binding to the immobilized DNA are delayed in their migration. Such a system has been used for example to construct complex DNA filters facilitating DNA computations. However, these gels are formed irreversibly and the choice of immobilized sequences is made once off during fabrication. In this work, we demonstrate the reversible self-assembly of gels combined with amphiphilic DNA molecules, which exhibit hydrophobic hydrocarbon chains attached to the nucleobase. This amphiphilic DNA, which we term lipid-DNA, is synthesized in advance and is blended into a block copolymer gel to induce sequence-dependent DNA retention during electrophoresis. Furthermore, we demonstrate and characterize the programmable mobility shift of matching DNA in such reversible gels both in thin films and microchannels using microelectrode arrays. Such sequence selective separation may be employed to select nucleic acid sequences of similar length from a mixture via local electronics, a basic functionality that can be employed in novel electronic chemical cell designs and other DNA information-processing systems.

The research work presented in this thesis was oriented on advanced thermoset materials and also on the conception of bio-based polymers and composites. This last topic has been investigated by the combination of different bio-based raw materials which are well-known to have a great potential to substitute the petroleum monomers. Firstly, a fundamental work has been done on the combination of epoxidized linseed oil (ELO) and anhydrides as cross-linkers, which links the polymerization reactivity with the network structure and thermomechanical properties. For being economically realistic, the bio-refineries are urged to valorize the sidestream products issued from biomass conversion. In that respect, a second study investigated successfully the incorporation and copolymerization of an important amount of humins (heterogeneous residues obtained during the sugar conversion into hydroxymethylfurfural (HMF)) with furfuryl alcohol (FA) in order to create new resins. Another proposed combination, focused on ELO and FA cationic copolymerization with the purpose to create new fully bio-based resins with tailored mechanical properties. Concerning the elaboration of advanced polymers and composites, a reflection around the hierarchically organized natural materials has been achieved in order to adapt the self-organization and structuration concepts to polymeric network.; Le travail de recherche présenté dans cette thèse s’est orienté vers l’élaboration de matériaux avancés et la conception de polymères/composites biosourcés. Ce dernier sujet a été entrepris à travers la combinaison de différentes matières premières biosourcées qui sont connues comme ayant un grand potentiel de substitution des monomères pétrosourcés. Tout d’abord, un travail fondamental a été exécuté en combinant l’huile de lin epoxydée (ELO) avec des dérivés d’anhydrides d’acides comme agents de réticulation, afin de relier la réactivité chimique de polymérisation à la structure du réseau formé et aux propriétés thermomécaniques. Afin de devenir économiquement viable, les bio-raffineries doivent urgemment valoriser les sous-produits issus de la conversion de la biomasse. Fort de ce constat, une deuxième étude sur l’incorporation et la copolymérisation d’une quantité importante d’humins (résidu hétérogène obtenu durant la conversion des sucres en hydroxymethylfurfural (HMF)) avec de l’alcool furfurylique (FA) a été réalisée avec succès afin de créer de nouvelles résines thermodures. Une autre voie proposée consiste en la combinaison de ELO et de FA à travers une polymérisation cationique, dans l’idée de créer de nouvelles résines totalement biosourcées, générant ainsi une gamme de matériaux aux propriétés mécaniques modulables.

Preliminary molecular dynamics simulations of the nematic phase of 4-n-pentyl-4'-cyanobiphenyl are described. The simulations include all molecular degrees of freedom. The influence of the molecular dipole moment is investigated by comparing simulations with and without a charge distribution on the molecules. Inclusion of the charge distribution is found to lead to a slight broadening of the orientational distribution function, in qualitative agreement with Raman measurements of the orientational order parameters.

The synthesis and mesomorphic properties of 30 bent-core compounds with a fluorine substituent in one of the outer rings are reported. The banana-shaped compounds are all derived from resorcinol and contain esters as linking groups between the five aromatic rings. The different mesophases have been characterized by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction studies and electro-optical investigations. The compounds with the longer terminal chains exhibit an antiferroelectric SmCP phase. Upon introduction of a fluorine substituent the layer spacing increases, as compared with the corresponding unsubstituted compound. The introduction of one terminal vinyl group in the mono-substituted bent-core mesogens has no significant influence on the liquid crystalline properties and the switching remains antiferroelectric. Due to the introduction of the terminal double bond, these banana-shaped compounds are suitable for the preparation of siloxane polymers or attachment to a hydrogen-terminated silicon surface

In this study, the binary phase diagram of two triphenylene derivatives is investigated by using differential scanning calorimetry (DSC), optical polarizing microscopy (OPM), and wide-angle x-ray scattering (WAXS). The triphenylene derivatives studied are based on hexa-alklyoxy-triphenylene. One compound in the binary mixture HAT6 has six symmetrical hexyloxy tails each containing six carbon atoms. The other compound, HAT6-C10UC6, has one asymmetrical tail bearing a urea functionality at a specific distance from the aromatic core. The introduction of this asymmetrical H-bonding unit into the structure results in a complete disruption of liquid crystallinity while a “pseudo re-entrant” phenomenon is observed in the binary mixture phase diagram.

We present a general approach based on nonequilibrium thermodynamics for bridging the gap between a well-defined microscopic model and the macroscopic rheology of particle-stabilised interfaces. Our approach is illustrated by starting with a microscopic model of hard ellipsoids confined to a planar surface, which is intended to simply represent a particle-stabilised fluid-fluid interface. More complex microscopic models can be readily handled using the methods outlined in this paper. From the aforementioned microscopic starting point, we obtain the macroscopic, constitutive equations using a combination of systematic coarse-graining, computer experiments and Hamiltonian dynamics. Exemplary numerical solutions of the constitutive equations are given for a variety of experimentally relevant flow situations to explore the rheological behaviour of our model. In particular, we calculate the shear and dilatational moduli of the interface over a wide range of surface coverages, ranging from the dilute isotropic regime, to the concentrated nematic regime.

A fifth-generation poly(propylene imine) dendrimer decorated with palmitoyl- and azobenzene-containing alkyl groups forms giant vesicles in aqueous solutions with diameters from 50 nm up to 20 mm and a multilaminar onion-like structure. Dense and ordered arrangement of the azobenzene chromophores in the bilayer structure leads to fluorescence with lmax= 600 nm. The fluorescence intensity can be increased by irradiation with blue light, and at low pH a distinctive blue shift of the spectrum is observed. With the aid of a single-beam optical tweezers it is possible to trap vesicles and direct them in a billiard-like fashion against each other using forces in the range of several pN. In collision experiments, the vesicles behave like hard spheres, and merging is not observed.

We report control over the phase behavior of CdS nanorods via the solvent and acidity. CdS nanorods were synthesized using alkane phosphonic acid ligands, which were replaced after synthesis by a series of aromatic ligands. Change of ligand enabled us to cast films from different solvents. By replacing toluene with ethanol or water the rod-rod interactions dominate over rod-substrate interactions, thereby favoring simple hexagonal ordering (2D). When dispersed in water, a net electrostatic charge on the nanorods could be induced by deprotonating the ligands at high pH. This net charge favors 2D nematic ordering over homeotropic ordering of the nanorods on a substrate. A calculation of the van der Waals and electrostatic interactions is presented that explains the observed influence of solvent and pH.

Templating approaches based on the self-assembly of block-copolymers represent a powerful tool for the preparation of ordered carbon-containing nanomaterials. Combining such strategies with the use of well-defined amphiphilic precursors with sp2-carbon-rich cores provides a way to better tailor carbon nanostructures with predetermined morphologies. This thesis, therefore, explores a soft templating approach using the self-assembly of linear poly(ethylene oxide)-substituted oligo(phenylene) amphiphiles as both a template for the formation of lamellar structures and sp2-carbon-rich source for the synthesis of ordered carbon nanostructures. Our investigations showed that a series of such coil-rod-coil molecules with different numbers of phenylene units in their core exhibited a rich polymorphism of tilted lamellar phases in bulk over a wide range of temperatures. We found that the self-assembly of the oligo(phenylene) rods into lamellar domains induced nanoscopic confinement of the poly(ethylene oxide) chains, resulting in a significant increase of their melting point to temperatures of up to 228 °C. Moreover, the cooperative self-assembly of the hexa(phenylene) amphiphile with sol-gel aluminosilicate precursors enhanced by solvent evaporation, and the subsequent carbonization of the obtained mesophases at 600 °C successfully furnished lamellar carbon-aluminosilicate nanocomposites. Following this approach, macroscopically oriented films of nanocomposites were prepared on a substrate by a shearing technique. Detailed structural characterization of the film morphology revealed lamellae with macroscopic order, an ¿edge-on¿ orientation relative to the substrate, as well as periodicities on the order of the molecular length scale. Furthermore, an ordered lamellar porous carbon was prepared by successfully etching the aluminosilicate phase, giving rise to a porous material with a large surface area that was appeared to be promising for hydrogen storage applications.

The critical interaction strength to induce mesomorphic structures in flexible polymers by complexing with surfactants is determined by using surfactants with different hydrogen-bonding strengths;. Two essential requirements have to be satisfied: (i) the association has to be strong enough, otherwise the structure dissociates. (ii) The polar-nonpolar repulsion has to be sufficiently strong to induce microphase separation. If the latter requirement is not satisfied, but the association is sufficiently strong, a characteristic comb polymer-like SAXS peak will still be present in the homogeneous melt state. To address these issues experimentally, poly(4-vinylpyridine) (P4VP) was combined with different classes of hydrogen-bonding surfactants: alkyl aromatic alcohols, alkyl carboxylic acids, alkyl aliphatic alcohols, and alkyl aliphatic amines. FTIR, SAXS, WAXS, and optical microscopy demonstrate that alkyl aromatic alcohols, such as 1-dodecyl 3,4,5-trihydroxybenzoate yield mesomorphic structures at not too high a temperature. Alkyl carboxylic acids are somewhat less effective. In the melt state, the SAXS data still show a peak, but because no birefringence could be found, the peak is attributed to block copolymer-like concentration fluctuations in an otherwise homogeneous state. Aliphatic alkyl alcohols yield still weaker hydrogen bonding. In the melt the short alkyl aliphatic alcohol surfactants form a homogeneous mixture with P4VP, but a SAXS peak is no longer observed. The longer ones macrophase separate. Both the alkyl carboxylic acids and the short alkyl aliphatic alcohols partly macrophase separate at lower temperatures due to crystallization.

We propose and numerically investigate an integrated optics refractometric nano-opto-fluidic sensor based on whispering gallery modes in sapphire microspheres. A measurand fluid is injected in a micromachined reservoir defined in between the microsphere and an optical waveguide. The wavelength shift due to changes in the refractive index of the measurand fluid are studied for a set of different configurations by the finite element method and a high sensitivity versus fluid volume is found. The proposed device can be tailored to work with a minimum fluid volume of 1 pl and a sensitivity up of 2000 nm/(RIU.nl). We introduce a figure of merit which quantifies the amplifying effect on the sensitivity of high quality factor resonators and allows us to compare different devices.

We study the deformation of a lipid membrane in response to a local pH modification. Experimentally, a basic solution is microinjected close to a giant unilamellar vesicle. A local deformation appears in the zone of the membrane that is closest to the micropipette, and relaxes when the injection is stopped. A theoretical description of this phenomenon is provided. It takes fully into account the spatiotemporal evolution of the concentration of hydroxide ions during and after the microinjection, as well as the linear dynamics of the membrane. This description applies to a local injection of any substance that reacts reversibly with the membrane lipids. We compare experimental data obtained in the domain of small deformations to the results of our linear description, and we obtain a good agreement between theory and experiments. In addition, we present direct experimental observations of the pH profile on the membrane during and after the microinjection, using pH-sensitive fluorescent lipids., Comment: 11 pages, 8 figures

11 pages; International audience; Using Fluorescence Recovery After Photobleaching, we investigate the Brownian motion of DNA rod-like fragments in two distinct anisotropic phases with a local nematic symmetry. The height of the measurement volume ensures the averaging of the anisotropy of the in-plane diffusive motion parallel or perpendicular to the local nematic director in aligned domains. Still, as shown in using a model specifically designed to handle such a situation and predicting a non-Gaussian shape for the bleached spot as fluorescence recovery proceeds, the two distinct diffusion coefficients of the DNA particles can be retrieved from data analysis. In the first system investigated (a ternary DNA-lipid lamellar complex), the magnitude and anisotropy of the diffusion coefficient of the DNA fragments confined by the lipid bilayers are obtained for the first time. In the second, binary DNA-solvent system, the magnitude of the diffusion coefficient is found to decrease markedly as DNA concentration is increased from isotropic to cholesteric phase. In addition, the diffusion coefficient anisotropy measured within cholesteric domains in the phase coexistence region increases with concentration, and eventually reaches a high value in the cholesteric phase.