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We experimentally explore the morphological evolution of cages in quasi-two-dimensional suspensions of colloidal fluids, uncovering a complex dynamic restructuring in the fluid. Although cages display isotropic evolution in the laboratory frame, we observe a striking anisotropy when analyzed in the displacement frame of the caged particles. Moreover, our findings reveal that particles in specific but distinct regions of the cage predominantly contribute to either its persistence or relaxation. Thus, our study provides a coarse-grained microscopic picture of the structural relaxation of these fluids through cage evolution, which has broader implications for the flow and phase behavior of complex fluids in confined geometry., Comment: 3 figures

Vapor deposition, known for precise structural control, is inevitably influenced by impurities. These impurities, often distinct from the depositing material, can significantly impact material properties, including local structure. Interestingly, the effect of immobilizing some of the depositing particles themselves, which would still preserve local structural symmetry, remains largely unexplored. By introducing a small fraction of pinned sites during colloidal vapor deposition and employing a combination of thermodynamic and kinematic measurements, we demonstrate that pinned sites, termed as "mobility impurities," are disfavored as nucleation centers. Moreover, tuning the mobility of mobile colloids can adjust nucleation likelihood. In later stages of growth, pinning induces mode localization, altering the thin film's vibrational spectrum. Our research thus underscores the potential of strategically incorporating mobility impurities to engineer material properties., Comment: 16 pages, 4 figures

Hydrodynamic interactions are important for diverse fluids especially those with low Reynold's number such as microbial and particle-laden suspensions, and proteins diffusing in membranes. Unfortunately, while far-field (asymptotic) hydrodynamic interactions are fully understood in two- and three-dimensions, near-field interactions are not, and thus our understanding of motions in dense fluid suspensions is still lacking. In this contribution, we experimentally explore the hydrodynamic correlations between particles in quasi-two-dimensional colloidal fluids in the near-field. Surprisingly, the measured displacement and relaxation of particle pairs in the body frame exhibit direction-dependent dynamics that can be connected quantitatively to the measured near-field hydrodynamic interactions. These findings, in turn, suggest a mechanism for how and when hydrodynamics can lead to a breakdown of the ubiquitous Stokes-Einstein relation (SER). We observe this breakdown, and interestingly, we show that the direction-dependent breakdown of the SER is ameliorated along directions where hydrodynamic correlations are smallest. In total, the work uncovers significant ramifications of near-field hydrodynamics on transport and dynamic restructuring of fluids in two-dimensions., Comment: 4 figures

Long-wavelength Mermin-Wagner fluctuations prevent the existence of translational long-range order, in two-dimensional systems at finite temperature. Their dynamical signature, which is the divergence of the vibrational amplitude with the system size, also affects disordered solids and washes out the transient solid-like response generally exhibited by liquids cooled below their melting temperature. Through a combined numerical and experimental investigation, here we show that long-wavelength fluctuations are also relevant at high temperature, where the liquid dynamics does not reveal a transient solid-like response. In this regime, they induce an unusual but ubiquitous decoupling between long-time diffusion coefficient $D$ and structural relaxation time $\tau$, where $D\propto \tau^{-\kappa}$, with $\kappa > 1$. Long-wavelength fluctuations have a negligible influence on the relaxation dynamics only at extremely high temperatures, in molecular liquids, or extremely low densities, in colloidal systems.

The monomer surface mobility is the single most important parameter that decides the nucleation density and morphology of islands during thin film growth. During template-assisted surface growth in particular, low surface mobilities can prevent monomers from reaching target sites and this results in a partial to complete loss of nucleation control. While in atomic systems a broad range of surface mobilities can be readily accessed, for colloids, owing to their large size, this window is substantially narrow and therefore imposes severe restrictions in extending template-assisted growth techniques to steer their self-assembly. Here, we circumvented this fundamental limitation by designing templates with spatially varying feature sizes, in this case moire patterns, which in the presence of short-range depletion attraction presented surface energy gradients for the diffusing colloids. The templates serve a dual purpose, first, directing the particles to target sites by enhancing their surface mean free paths and second, dictating the size and symmetry of the growing crystallites. Using optical microscopy, we directly followed the nucleation and growth kinetics of colloidal islands on these surfaces at the single-particle level. We demonstrate nucleation control, with high fidelity, in a regime that has remained unaccessed in theoretical, numerical and experimental studies on atoms and molecules as well. Our findings pave the way for fabricating non-trivial surface architectures composed of complex colloids and nanoparticles., Comment: 12 pages, 3 figures

We examine the role of shape of dynamical heterogeneities on the validity of the Stokes-Einstein (SE) and Stokes-Einstein-Debye (SED) relations in quasi-two-dimensional suspensions of colloidal ellipsoids. For ellipsoids with repulsive interactions, although the orientational relaxation time remains coupled to the structural one, the SED relation by the Einstein formalism shows a breakdown. Strikingly, we find that it is the change in the shape of the dynamical heterogeneities from string-like to compact and not just their presence that results in the breakdown of both the SE and SED relations. On introducing a short-range depletion attraction between the ellipsoids, associated with the lack of morphological evolution of dynamical heterogeneities, the SE and SED relations remain valid even for deep supercooling. Our observations are consistent with numerical predictions., Comment: 11 pages, 3 figures

One of the greatest challenges in contemporary condensed matter physics is to ascertain whether the formation of glasses from liquids is fundamentally thermodynamic or dynamic in origin. While the thermodynamic paradigm has dominated theoretical research for decades, the purely kinetic perspective of the dynamical facilitation (DF) theory has attained prominence in recent times. In particular, recent experiments and simulations have highlighted the importance of facilitation using simple model systems composed of spherical particles. However, an overwhelming majority of liquids possess anisotropy in particle shape and interactions and it is therefore imperative to examine facilitation in complex glass-formers. Here, we apply the DF theory 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 heterogeneities. 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., Comment: 22 pages, 3 main figues, 3 supplementary figures. Submitted to Proceedings of the National Academy of Sciences, USA, on the 15th of July, 2014