Deconstructing the glass transition through critical experiments on colloids

The glass transition is the most enduring grand-challenge problem in contemporary condensed matter physics. Here, we review the contribution of colloid experiments to our understanding of this problem. First, we briefly outline the success of colloidal systems in yielding microscopic insights into a wide range of condensed matter phenomena. In the context of the glass transition, we demonstrate their utility in revealing the nature of spatial and temporal dynamical heterogeneity. We then discuss the evidence from colloid experiments in favor of various theories of glass formation that has accumulated over the last two decades. In the next section, we expound on the recent paradigm shift in colloid experiments from an exploratory approach to a critical one aimed at distinguishing between predictions of competing frameworks. We demonstrate how this critical approach is aided by the discovery of novel dynamical crossovers within the range accessible to colloid experiments. We also highlight the impact of alternate routes to glass formation such as random pinning, trajectory space phase transitions and replica coupling on current and future research on the glass transition. We conclude our review by listing some key open challenges in glass physics such as the comparison of growing static lengthscales and the preparation of ultrastable glasses, that can be addressed using colloid experiments.Comment: 137 pages, 45 figure

Site-Specific Colloidal Crystal Nucleation by Template-enhanced Particle Transport

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 figure

Strength of Mechanical Memories is Maximal at the Yield Point of a Soft Glass

We show experimentally that both single and multiple mechanical memories can be encoded in an amorphous bubble raft, a prototypical soft glass, subject to an oscillatory strain. In line with recent numerical results, we find that multiple memories can be formed sans external noise. By systematically investigating memory formation for a range of training strain amplitudes spanning yield, we find clear signatures of memory even beyond yielding. Most strikingly, the extent to which the system recollects memory is largest for training amplitudes near the yield strain and is a direct consequence of the spatial extent over which the system reorganizes during the encoding process. Our study further suggests that the evolution of force networks on training plays a decisive role in memory formation in jammed packings.Comment: 13 pages, 4 Figure

Growing Dynamical Facilitation on Approaching the Random Pinning Colloidal Glass Transition

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 observations of growing length scales are consistent with thermodynamic perspectives like the Random First-Order Transition theory (RFOT), the purely dynamic approach of the Dynamical Facilitation (DF) theory lacks experimental validation. Further, for glass transitions induced by randomly freezing a subset of particles in the liquid phase, simulations support the predictions of RFOT, whereas the DF theory remains unexplored. Here, using video microscopy and holographic optical tweezers, we show that dynamical facilitation in a colloidal glass-forming liquid unambiguously grows with density as well as the fraction of pinned particles. In addition, we show that heterogeneous dynamics in the form of string-like cooperative motion, which is believed to be consistent with RFOT, emerges naturally within the framework of facilitation. Most importantly, our findings demonstrate that a purely dynamic origin of the glass transition cannot be ruled out.Comment: 13 pages, 3 figures. Submitted to Nature Communications on the 17th of March, 201

Direct measurements of growing amorphous order and non-monotonic dynamic correlations in a colloidal glass-former

While the transformation of flowing liquids into rigid glasses is omnipresent, a complete understanding of vitrification remains elusive. Of the numerous approaches aimed at solving the glass transition problem, the Random First-Order Theory (RFOT) is the most prominent. However, the existence of the underlying thermodynamic phase transition envisioned by RFOT remains debatable, since its key microscopic predictions concerning the growth of amorphous order and the nature of dynamic correlations lack experimental verification. Here, by using holographic optical tweezers, we freeze a wall of particles in an equilibrium configuration of a 2D colloidal glass-forming liquid and provide direct evidence for growing amorphous order in the form of a static point-to-set length. Most remarkably, we uncover the non-monotonic dependence of dynamic correlations on area fraction and show that this non-monotonicity follows directly from the change in morphology of cooperatively rearranging regions, as predicted by RFOT. Our findings suggest that the glass transition has a thermodynamic origin

Influence of an amorphous wall on the distribution of localized excitations in a colloidal glass-forming liquid

Elucidating the nature of the glass transition has been the holy grail of condensed matter physics and statistical mechanics for several decades. A phenomenological aspect that makes glass formation a conceptually formidable problem is that structural and dynamic correlations in glass-forming liquids are too subtle to be captured at the level of conventional two-point functions. As a consequence, a host of theoretical techniques, such as quenched amorphous configurations of particles, have been devised and employed in simulations and colloid experiments to gain insights into the mechanisms responsible for these elusive correlations. Very often, though, the analysis of spatio-temporal correlations is performed in the context of a single theoretical framework, and critical comparisons of microscopic predictions of competing theories are thereby lacking. Here, we address this issue by analysing the distribution of localized excitations, which are building blocks of relaxation as per the Dynamical Facilitation (DF) theory, in the presence of an amorphous wall, a construct motivated by the Random First-Order Transition theory (RFOT). We observe that spatial profiles of the concentration of excitations exhibit complex features such as non-monotonicity and oscillations. Moreover, the smoothly varying part of the concentration profile yields a length scale $\xi_c$, which we compare with a previously computed length scale $\xi_{dyn}$. Our results suggest a method to assess the role of dynamical facilitation in governing structural relaxation in glass-forming liquids.Comment: 19 pages, 7 figure

Growing Surface Tension of Amorphous-Amorphous Interfaces on Approaching the Colloidal Glass Transition

There is mounting evidence indicating that relaxation dynamics in liquids approaching their glass transition not only becomes increasingly cooperative (1,2) but the relaxing regions also become more compact in shape(3-7). While the surface tension of the interface separating neighboring relaxing regions is thought to play a crucial role in deciding both their size and morphology(8-10), owing to the amorphous nature of these regions, even identifying these interfaces has not been possible in bulk liquids. Here, by devising a scheme to identify self-induced disorder sites in bulk colloidal liquids, we directly quantified the dynamics of interfaces delineating regions of high and low configurational overlap. This procedure also helped unveil a non-monotonicity in dynamical correlations that has never been observed in bulk supercooled liquids. Using the capillary fluctuation method (11,12), we measured the surface tension of amorphous-amorphous interfaces with supercooling and find that it increases rapidly across the mode-coupling area fraction. Remarkably, a similar growth in the surface tension is also seen in the presence of a pinned amorphous wall. Our observations help prune theories of glass formation and opens up new research avenues aimed at tuning the properties of amorphous-amorphous interfaces, and hence the glass itself, in a manner analogous to grain boundary engineering in polycrystals (13)

Nonlinear flow of wormlike micellar gels: regular and chaotic time-dependence of stress, normal force and nematic ordering

We present our recent experiments on the nonlinear dynamics in the flow behaviour of shear-thinning wormlike micellar gels. In particular, we have shown experimentally that above a critical shear rate, the initial transient stress response comprises of overshoots and undershoots and this is followed by periodic, quasiperiodic, intermittent and chaotic behaviour. The normal force dynamics is similar to that of the stress. This can be classified as Type-II intermittency route to chaos. In our system, shear-thinning wormlike micellar solution of cetyltrimethylammonium tosylate, the strength of flow-concentration coupling is tuned by the addition of salt sodium chloride. The existence of a "butterfly" intensity pattern in small angle light scattering (SALS) performed simultaneously with the rheological measurements confirms the coupling of flow to concentration fluctuations in the system under study. Dynamic light scattering measurements to extract the relaxation time scales of concentration fluctuations are in excellent agreement with the in situ SALS measurements performed under shear. The scattered depolarised intensity in SALS, sensitive to orientational order fluctuations, shows the same time-dependence (like intermittency) as that of shear stress at various wave vectors