1,581 research outputs found
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
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
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
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)
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
, which we compare with a previously computed length scale .
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
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
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