Stable Frank-Kasper spherical packing phases have been observed in a wide variety of soft-condensed matter systems, but the universality of these phases is not well understood. Recently, it was shown that the Frank-Kasper $\sigma$ and A15 phases are stable in the well-known Landau-Brazovskii (LB) model. In this work we consider the $\sigma$ and A15 phases, as well as the Laves C14 and C15 phases, and show that none of these is stable in the Ohta-Kawasaki (OK) model, which is another widely studied Ginzburg-Landau theory. The LB and OK models differ only in their quadratic coefficients. We conduct a thorough investigation of the role that this coefficient plays in stabilizing the complex phases. We uncover generic principles linking the functional form of the coefficient in reciprocal space with the stability of the complex phases. A Ginzburg-Landau theory for a for diblock copolymer system with a conformational asymmetry parameter is derived, but the complex phases are not found to be stable in this model. We also consider a Ginzburg-Landau theory for a system of hard spheres interacting via a pairwise short-range attractive, long-range repulsive (SALR) potential, and use our framework to demonstrate how the parameters in the potential influence the stability of the Frank-Kasper phases. Taken together, these results provide insight into the universal mechanisms that underlie the formation of the complex spherical packing phases in soft condensed matter. / Thesis / Doctor of Philosophy (PhD) / Soft condensed matter physics is the study of soft, deformable materials, such as soap bubbles, foams, and plastics. Many different soft matter systems undergo a fascinating phenomenon known as self-assembly, wherein the constituent particles spontaneously arrange themselves to form various ordered structures. In particular, the spherical packing phases appear when the particles first cluster into spherical aggregates, which then pack into larger arrangements. This sort of self-assembly is interesting because many different spherical arrangements are observed, including the complex spherical packing phases (also known as the Frank-Kasper phases). The fact that these complex phases appear in many different types of materials is not well understood. In this thesis we use a model known as the Ginzburg-Landau theory to ask which of these arrangements will form in a given system, and why. We uncover generic features of the Ginzburg-Landau theory that control which spherical packing phases appear, and we connect these features to several specific systems. These results provide insight into the mechanisms behind the formation of the complex spherical packing phases in a diverse range of systems.

Thesis (Ph. D.)--University of Chicago, Department of Physics. / Includes bibliographical references. Also available on the Internet.

The properties of conventional materials are inextricably linked with their molecular composition; to make water flow like wine would require changing its molecular identity. To circumvent this restriction, I have contstructed and characterized a two-dimensional metafluid, so-called because its constitutive dynamics are derived not from atoms and molecules but from macroscopic, chaotic surface waves excited on a vertically agitated fluid. Unlike in conventional fluids, the viscosity and temperature of this metafluid are independantly tunable. Despite this unconventional property, our system is surprisingly consistent with equilibrium thermodynamics, despite being constructed from macroscopic, non-equilibrium elements. As a programmable material, our metafluid represents a new platform on which to study complex phenomena such as self-assembly and pattern formation. We demonstrate one such application in our study of short-chain polymer analogs embedded in our system.

No description available.

A theoretical formalism to reconstruct structural and dynamical properties of polymer liquids from their coarse-grained description is developed. This formalism relies on established earlier analytical coarse-graining of polymers derived from the first principles of liquid theory. The polymer chain is represented at a mesoscale level as a soft particle. Coarse-grained computer simulations provide input data to the reconstruction formalism and allow one to achieve the most gain in computational efficiency. The structure of polymer systems is reconstructed by combining global information from mesoscale simulations and local information from small united-atom simulations. The obtained monomer total correlation function is tested for a number of systems including polyethylene melts of different degrees of polymerization as well as melts with different local chemical structure. The agreement with full united-atom simulations is quantitative, and the procedure remains advantageous in computational time. The dynamics in mesoscale simulations is artificially accelerated due to the coarse-graining procedure and needs to be rescaled. The proposed formalism addresses two rescalings of the dynamics. First, the internal degrees of freedom averaged out during coarse-graining procedure are reintroduced in "a posteriori" manner, rescaling the simulation time. The second rescaling takes into account the change in friction when switching from a monomer level description to mesoscopic. Both friction coefficients for monomer and soft particle are calculated analytically and their ratio provides the rescaling factor for the diffusion coefficient. The formalism is extensively tested against the united-atom molecular dynamic simulations and experimental data. The reconstructed diffusive dynamics of the center-of-mass for polyethylene and polybutadiene melts of increasing degrees of polymerization show a quantitative agreement, supporting the foundation of the approach. Finally, from the center-of-mass diffusion the monomer friction coefficient is obtained and used as an input into Cooperative Dynamics theory. The dynamics of polymer chains at any length scale of interest is described through a Langevin equation. In summary, the proposed formalism reconstructs the structure and dynamics of polymer melts enhancing computational efficiency of molecular dynamic simulations. This dissertation includes previously published and unpublished co-authored material.

A novel colloidal model system is developed for the purpose of achieving simultaneous three-dimensional (3D) confocal imaging and optical tweezing of impurity probe particles embedded in a dense material of colloidal host particles. First, the colloidal host particles are developed from 3-trimethoxysilyl-propylmethacrylate (TPM), and the solvent mixture is tuned to match the refractive index and the density of the particles. A sedimentation-diffusion equilibrium profile of the TPM particles was imaged in 3D to establish suitability of the system for 3D confocal laser scanning microscopy, and to study its phase behaviour and particle dynamics. Then, core-shell particles, which consist of a high refractive index core and a TPM shell, are synthesised to be used as the impurities. The versatility of the two-step coating procedure with TPM has been demonstrated on several core materials, and the optical properties of the core-shell particle are established using digital holographic microscopy and their optical trapping strengths. Together with the TPM host particles, the core-shell particles are shown to be suitable impurity probes in dense colloidal materials, as they can be manipulated using optical tweezers in all three-dimensions, whilst the structure and dynamics of the surrounding host particles can be imaged simultaneously using fast confocal laser scanning microscopy. Subsequently, to demonstrate the capability of the TPM-based colloidal model system, the depletion potential of a pair of core-shell probe particles embedded in a sea of TPM host particles has been measured using optical tweezing. This is derived from comparing the direct pair potential between the core-shell particles in a TPM refractive index matching solvent, and the potential of mean force for the core-shell particles embedded in a dense fluid region of the TPM host particles. Direct visualisation of the liquid structures of the TPM host particles in the binary system around the probe particles has been linked to the form of the depletion potential, and its oscillatory nature as a function of the particle separation. Lastly, the formation of colloidal dumbbell particles is discussed. The dumbbell formation has been rationalised in terms of the total surface energy of droplet formation on spherical surfaces and the calculations are compared with experimental results from coating various seed particles with TPM. Using optically anisotropic dumbbell particles and tuning the refractive index of the dispersion medium, a preliminary two-trap experiment has been conducted which shows unusual trapping behaviour when a time-delayed feedback trapping trajectory has been applied.

The importance of soft matter research, as a major class of materials including liquid crystals, polymers, colloids, emulsions, and forms, is attributed to the behavior resemblances in each branch of soft matter responding to the external perturbations. Hence, one of the most required inquiries in soft matter physics is understanding how the structures with characteristic length scales evolve in response to external perturbations, and concomitant phase transitions. We have focused on adopting polarized Raman spectroscopy to probe phase transitions in soft materials consisting of anisometric components and the evolution of molecular orientational ordering as a complementary tool to other methodologies, but distinct in some respects. The primary task is quantifying the degree of molecular orientation, i.e., obtaining orientational order parameters, in liquid crystal (LCs) system. Thermal evolution of orientation degree in a hitherto elusive biaxial nematic (Nb) phase as well as a commonly known uniaxial nematic (Nu) phase were interrogated from the measurements of anisotropy in polarized Raman intensities. We demonstrated reliable and applicable method to quantify the orientation degree for systems possessing anisotropic ordering. We also addressed a strong potential of Raman spectroscopy that the changes of vibrational energy reflect the variations of intermolecular interactions and structural changes on the molecular level induced by phase transitions. As a subfield of soft matter, we characterized phase transitions and anisotropic ordering observed in an evaporating conjugated polymer solution and elucidated the mechanism of the entities undergoing phase transitions using mainly polarized Raman spectroscopy. In addition, we have shown that tracking Raman spectral changes can provide valuable information for understanding structure-property relations when the measurements of the evolution in physical properties are carried out simultaneously.

Le confinement nanométrique permet d'obtenir la frustration des fluctuations et/ou des transitions de phases spontanées qu'un fluide moléculaire présente en volume (i.e. en " bulk "). Le confinement au sein de matrices poreuses est donc une voie usuelle de stabilisation de phases métastables. Nous détaillons ici les propriétés structurales, dynamiques et thermodynamiques de deux systèmes moléculaires confinés : dans un premier chapitre, nous nous intéressons au cas de l'eau puis, dans un deuxième chapitre, nous traitons le cas spécifique d'un polymère semi-cristallin. Le confinement permet d'abaisser considérablement le point de fusion du fluide confiné. Cette propriété a été récemment mise à profit dans la cadre de nombreux travaux visant à tester l'existence d'un hypothétique point critique à basse température dans l'eau volumique à 228 K and 100 MPa. Dans cette contribution, nous mettons en évidence des propriétés dynamiques surprenantes de l'eau interfaciale à basse température (de 100 à 300 K). Nous proposons un modèle de percolation décrivant les transitions dynamiques et thermodynamiques que nous observons à 150, 220 et 240 K. Nous proposons une description cohérente de cette eau à deux dimensions et de ses propriétés. Nous invoquons le rôle dominant des interactions de surface pour remettre en cause la pertinence de l'utilisation de l'eau confinée pour prouver l'existence d'un point critique à basse température dans l'eau volumique. Cette étude met cependant en évidence l'existence d'une transition liquide-liquide (l'une des conditions pour observer un point critique) impliquant des molécules d'eau. Récemment, un " effet corset " a été proposé : le confinement induirait une réduction d'un ordre de grandeur du diamètre du tube de reptation d'un polymère (quelques nanomètres en volume contre quelques angströms sous confinement). Dans le second chapitre, nous utilisons une approche par diffusion de neutrons pour accéder à une description multi-échelles de la dynamique d'un polymère (en volume puis sous confinement) de l'échelle atomique à temps court (picosecondes) jusqu'à une dizaine de nanomètres, à temps long (600 nanosecondes). Cette étude détaillée de la dépendance spatiale de la relaxation temporelle des chaînes de polymère ne permet pas de mettre en évidence d'"effet corset ". De façon générale, lorsque l'on cherche à tirer profit du confinement nanométrique pour obtenir des "effets de volume", en plus d'"effets de surface" parasites décrits dans le premier chapitre, on est également confronté à une perte significative d'information induite par la moyenne spatiale des observables spectroscopiques. Nous décrivons dans le deuxième chapitre comment utiliser des matrices de confinement orientées macroscopiquement pour s'affranchir de ces effets indésirables et/ou limitants. Dans le troisième et dernier chapitre, nous définissons un système de confinement nanométrique qui permet d'associer i) une orientation macroscopique des pores et ii) une absence totale d'interactions de surface. Un tel système permet d'envisager des effets de volume unidimensionnels très significatifs et ayant une portée sur des distances macroscopiques. Nous discutons pourquoi de tels " tuyaux nanométriques" peuvent potentiellement intéresser à la fois la recherche fondamentale et l'industrie.

Ce manuscript d'Habilitation à Diriger des Recherches présente mes travaux de recherche pour la période 2000-2011, pendant laquelle je me suis intéressée à la description et à la compréhension de la dynamique d'écoulement des mousses aqueuses ainsi qu' à l'écoulement de liquide et de particules dans un tel milieu. Une brève introduction aux mousses aqueuses dé finit le matériau et ses propriétés caractéristiques ainsi qu'un vocabulaire parfois spéci fique à la communauté "mousse". Les problèmatiques et leur contexte sont présentés plus précisément au début de chaque chapitre ou sous-chapitre.

Cet ouvrage représente un résumé partiel de mes travaux depuis l'arrivée au Laboratoire de Physique des Solides en octobre 2005, en tant que chargé de recherches au CNRS. Le projet que j'avais présenté à l'époque portait principalement sur la formulation et l'étude par des techniques de diffusion du rayonnement de phases lamellaires de tensioactifs dopées avec des inclusions inorganiques, sphériques ou anisotropes. Un deuxième sujet de recherche (dans la même mouvance que le premier) envisageait la mesure de l'interaction entre des peptides insérés dans des membranes de lipides. Finalement, un troisième sujet assez distinct des deux précédents était l'étude de la dynamique de particules colloïdales par la diffusion dynamique des rayons X (XPCS, selon l'acronyme anglais).