Les suspensions colloïdales de particules sphériques présentant des interactions de type sphères dures font partie des systèmes les plus simples et les plus largement étudiés en Matière Molle. Elles peuvent être considérées comme systèmes modèles pour tester des théories plus générales, par exemple en ce qui concerne la cristallisation [1] ou la transition vitreuse [2]. Malgré de nombreux résultats théoriques et expérientaux dans ce domaine, le comportement dynamique des suspensions de sphères dures n'a pas été complètement élucidé.La spectroscopie à corrélation de photons X (XPCS) est une technique de diffusion cohérente équivalente à la Diffusion Quasi-Elastique de la Lumière [3], qui est un des principaux outils d'investigation de la dynamique colloïdale [4]. Comparée à la luière visible, l'utilisation de rayons X procure des rensignements sur les transferts de moment de plus haute énergie, et évite les diffusions multiples - phénomène qui complique sensiblement les études en DQEL pour les échantillons concentrés. De plus, l'utilisation du détecteur 2D compteur de photons (MAXIPIX) disponible sur la ligne ID10 (ESRF) donne des renseignements sur l'évolution de la dynamique de l'échantillon au cours de l'exposition, via les fonctions de corrélation à deux temps.Dans ce travail, nous avons étudié une suspension de spheres colloïdales de PMMA (poly(méthylmétacrylate)) stériquement stabilisées. La distribution en taille des particules et leur concentration ont été obtenues par diffusion de rayons X aux petits angles (SAXS). Les expériences de XPCS effectuées aux plus grandes fractions volumiques en particules (>0.5) mettent en évidence à la fois des temps de diffusion courts et des temps longs autour des pics de Bragg. Une comparaison avec une précédente étude [5] montre, pour une petite gamme de fractions volumiques, une modification drastique de la loi d'échelle entre les temps de relaxation courts et les temps longs qui avait été initialement proposée par Segrè et Pusey [6]. L'analyse des fonctions de corrélation à deux temps révèle un comportement dynamique complexe des échantillons légèrement au-dessus de la transition vitreuse, alors qu'on n'observe aucun signe de modifications structurales via diffusion statique. Utiliser la XPCS sur des suspensions en écoulement dans des canaux cylindriques avait fait ses preuves pour renseigner à la fois sur les propriétés dynamiques et d'écoulement de suspensions diluées [7]. Ici, nous discutons les potentialités et les limites de cette méthode, en étudiant l'interaction entre les propriétés rhéologiques et dynamiques dans ces systèmes complexes modèles que sont les verres colloïdaux.[1] P. N. Pusey and W. van Megen. In: Nature 320.6060 (Mar. 1986), pp. 340–342 [2] P. N. Pusey and W. van Megen. In: Phys. Rev. Lett. 59 (18 1987), pp. 2083–2086.[3] V. A. Martinez et al. In: The Journal of Chemical Physics 134.5, 054505 (2011), p. 054505.[4] B. J. Berne and R. Pecora. Dynamic Light Scattering with application to chemistry, biology and physics. Dover Publications, New York, 2000. [5] D. Orsi et al. “Dynamics in dense hard-sphere colloidal suspensions”. In: Phys. Rev. E 85 (1 2012), p. 011402. doi: 10.1103/PhysRevE.85.011402. url: http://link.aps.org/doi/1 0.1103/PhysRevE.85.011402. [6] P. N. Segrè and P. N. Pusey. In: Phys. Rev. Lett. 77.4 (1996), pp. 771–774.[7] A. Fluerasu et al. In: New Journal of Physics 12.3 (2010) / Colloidal suspensions of spherical particles presenting hard-sphere like interactions is one of the simplest and most widely studied systems of soft condensed matter. They can be treated as a model for testing fundamental theories, regarding e.g. crystallization [1] or glass transition [2]. Despite the long history of both theoretical and experimental research, the dynamic behavior of hard sphere suspensions still lacks a complete understanding.X-ray Photon Correlation Spectroscopy (XPCS) is a coherent scattering technique equivalent to Dynamic Light Scattering (DLS) [3], which is one of the main tools used in the study of colloidal dynamics [4]. Comparing to visible light, the use of X-rays provides access to higher momentum transfer vector values and allows to avoid multiple scattering – a phenomena significantly complicating DLS measurements on concentrated samples. Moreover, the use of a fast, single photon counting area detector (MAXIPIX) available at the ID10 beamline at ESRF gives insight into the evolution of sample dynamics during the measurement time by the means of two-time correlation functions.In this work suspensions of sterically stabilized poly(methyl methacrylate) (PMMA) colloidal spheres were used. Particle size, polydispersity and volume fractions of the samples were obtained using the Small-Angle X-ray Scattering (SAXS) technique. XPCS measurements at high volume fractions (>0.5) show both short- and long-time diffusive behaviour for scattering vector values around, but not restricted to the structure factor peak position. A comparison with an earlier study [5] shows a dramatic change in the approximate scaling between the short- and long-time relaxation rates, initially proposed by Segrè and Pusey in [6], over a small range of volume fractions. The analysis of two-time correlation functions reveals complex dynamic behaviour of a sample slightly above the glass transition, while no signs of structural changes are observed in the static scattering patterns. The studies indicate the dynamics being governed by a jamming transition driven by restrictions in free volume rather than a glass transition as know from the mode-coupling theory. A combination of XPCS with flow in a cylindrical channel has demonstrated previously to give both dynamic and flow properties of dilute suspensions [7]. Here we discuss the potential and limitations of this method in the study of the interplay between rheological properties and dynamics in complex systems such as colloidal glasses. [1] P. N. Pusey and W. van Megen. In: Nature 320.6060 (Mar. 1986), pp. 340–342[2] P. N. Pusey and W. van Megen. In: Phys. Rev. Lett. 59 (18 1987), pp. 2083–2086.[3] V. A. Martinez et al. In: The Journal of Chemical Physics 134.5, 054505 (2011), p. 054505.[4] B. J. Berne and R. Pecora. Dynamic Light Scattering with application to chemistry, biology and physics. Dover Publications, New York, 2000.[5] D. Orsi et al. “Dynamics in dense hard-sphere colloidal suspensions”. In: Phys. Rev. E 85 (2012), p. 011402.[6] P. N. Segrè and P. N. Pusey. In: Phys. Rev. Lett. 77.4 (1996), pp. 771–774.[7] A. Fluerasu et al. In: New Journal of Physics 12.3 (2010)

The work presented in this doctoral thesis employs colloidal suspensions to explore key open problems in condensed matter physics. Colloidal suspensions, along with gels, polymers, emulsions and liquid crystals belong to a family of materials that are collectively labelled as soft matter. Compositionally, colloidal suspensions consist of particles whose size ranges from a few nanometers to a few microns, dispersed in a solvent. A hallmark feature of these systems is that they exhibit Brownian motion, which makes them suitable for investigating statistical mechanical phenomena. Over the last fifteen years or so, colloids have been used extensively as model systems to shed light on a wide array of such phenomena typically observed in atomic systems. The chief reason why colloids are good mimics of atomic systems is their large size and slow dynamics. Unlike atomic systems, the dynamics of colloids can be probed in real time with single-particle resolution, which allows one to establish the link between macroscopic behavior and the microscopic processes that give rise to it. Yet another important feature is that colloidal systems exhibit various phases of matter such as crystals, liquids and glasses, which makes them versatile model systems that can probe a broad class of condensed matter physics problems. The work described in this thesis takes advantage of these lucrative features of colloidal suspensions to gain deeper insights into the physics of glass formation as well as shear-induced anisotropic grain growth in polycrystalline materials. The thesis is organized into two preliminary chapters, four work chapters and a concluding chapter, as follows. Chapter 1 provides an introduction to colloidal suspensions and reviews the chief theo-retical concepts regarding glass formation and grain boundary dynamics that form an integral part of subsequent chapters. Chapter 2 describes the experimental methods used for performing the work presented in the thesis and consists of two parts. The first part describes the protocols followed for synthesizing the size-tunable poly (N-isoprolypacrylamide) (PNIPAm) particles used in our study of shear-induced grain growth. The second part describes the instrumentation and techniques, such as holographic optical tweezers, confocal microscopy, rheology and Bragg diffraction microscopy, used to perform the measurements described in the thesis. Chapter 3 deals with our work on the dynamical facilitation (DF) theory of glass forma-tion. Despite decades of research, it remains to be established whether the transformation of a liquid into a glass is fundamentally thermodynamic or dynamic in origin. While obser-vations of growing length scales are consistent with thermodynamic perspectives, the purely dynamic approach of the DF theory has thus far lacked experimental support. Further, for glass transitions induced by randomly freezing a subset of particles in the liquid phase, theory and simulations support the existence of an underlying thermodynamic phase transi-tion, whereas the DF theory remains unexplored. In Chapter 3, using video microscopy and holographic optical tweezers, we show that dynamical facilitation in a colloidal glass-forming liquid grows with density as well as the fraction of pinned particles. In addition, we observe that heterogeneous dynamics in the form of string-like cooperative motion, which is consid-ered to be consistent with thermodynamic theories, can also emerge naturally within the framework of facilitation. These findings suggest that a deeper understanding of the glass transition necessitates an amalgamation of existing theoretical approaches. In Chapter 4, we further explore the question of whether glass formation is an intrinsi-cally thermodynamic or dynamic phenomenon. A major obstacle in answering this question lies in determining whether relaxation close to the glass transition is dominated by activated hopping, as espoused by various thermodynamic theories, or by the correlated motion of localized excitations, as proposed in the Dynamical Facilitation (DF) approach. In Chapter 4, we surmount this central challenge by developing a scheme based on real space micro-scopic analysis of particle dynamics and applying it to ascertain the relative importance of hopping and facilitation in a colloidal glass-former. By analysing the spatial organization of excitations within cooperatively rearranging regions (CRRs) and examining their parti-tioning into shell-like and core-like regions, we establish the existence of a crossover from a facilitation-dominated regime at low area fractions to a hopping-dominated one close to the glass transition. Remarkably, this crossover coincides with the change in morphology of CRRs predicted by the Random First-Order Transition theory (RFOT), a prominent ther-modynamic framework. Further, we analyse the variation of the concentration of excitations with distance from an amorphous wall and find that the evolution of these concentration profiles with area fraction is consistent with the presence of a crossover in the relaxation mechanism. By identifying regimes dominated by distinct dynamical processes, our study offers microscopic insights into the nature of structural relaxation close to the glass transi-tion. In Chapter 5, we extend our investigation of the glass transition to systems composed of anisotropic particles. The primary motivation for this is to bridge a long-standing di-vide between theories and simulations on one hand, and experiments on molecular liquids on the other. In particular, theories and simulations predominantly focus on simple glass-formers composed of spherical particles interacting via isotropic interactions. Indeed, even the prominent theory of Dynamical Facilitation has not even been formulated to account for anisotropic shapes or interactions. On the other hand, an overwhelming majority of liquids possess considerable anisotropy, both in particle shape as well as interactions. In Chapter 5, we mitigate this situation by developing the DF theory further and applying it to systems with orientational degrees of freedom as well as anisotropic attractive interactions. By analyzing data from experiments on colloidal ellipsoids, we show that facilitation plays a pivotal role in translational as well as orientational relaxation. Further, we demonstrate that the introduction of attractive interactions leads to spatial decoupling of translational and rotational facilitation, which subsequently results in the decoupling of dynamical het-erogeneities. Most strikingly, the DF theory can predict the existence of reentrant glass transitions based on the statistics of localized dynamical events, called excitations, whose duration is substantially smaller than the structural relaxation time. Our findings pave the way for systematically testing the DF approach in complex glass-formers and also establish the significance of facilitation in governing structural relaxation in supercooled liquids. In Chapter 6, we turn our attention away from the glass transition and address the problem of grain growth in sheared polycrystalline materials. The fabrication of functional materials via grain growth engineering implicitly relies on altering the mobilities of grain boundaries (GBs) by applying external fields. While computer simulations have alluded to kinetic roughening as a potential mechanism for modifying GB mobilities, its implications for grain growth have remained largely unexplored owing to difficulties in bridging the disparate length and time scales involved. In Chapter 6, by imaging GB particle dynamics as well as grain network evolution under shear, we present direct evidence for kinetic roughening of GBs and unravel its connection to grain growth in driven colloidal polycrystals. The capillary fluctuation method allows us to quantitatively extract shear-dependent effective mobilities. Remarkably, our experiments reveal that for sufficiently large strains, GBs with normals parallel to shear undergo preferential kinetic roughening resulting in anisotropic enhancement of effective mobilities and hence directional grain growth. Single-particle level analysis shows that the anisotropy in mobility emerges from strain-induced directional enhancement of activated particle hops normal to the GB plane. Finally, in Chapter 7, we present our conclusions and discuss possible future directions.