26 research outputs found
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
Brief Announcement: Fast and Scalable Group Mutual Exclusion
The group mutual exclusion (GME) problem is a generalization of the classical mutual exclusion problem in which every critical section is associated with a type or session. Critical sections belonging to the same session can execute concurrently, whereas critical sections belonging to different sessions must be executed serially. The well-known read-write mutual exclusion problem is a special case of the group mutual exclusion problem.
In a shared memory system, locks based on traditional mutual exclusion or its variants are commonly used to manage contention among processes. In concurrent algorithms based on fine-grained synchronization, a single lock is used to protect access to a small number of shared objects (e.g., a lock for every tree node) so as to minimize contention window. Evidently, a large number of shared objects in the system would translate into a large number of locks. Also, when fine-grained synchronization is used, most lock accesses are expected to be uncontended in practice.
Most existing algorithms for the solving the GME problem have high space-complexity per lock. Further, all algorithms except for one have high step-complexity in the uncontented case. This makes them unsuitable for use in concurrent algorithms based on fine-grained synchronization. In this work, we present a novel GME algorithm for an asynchronous shared-memory system that has O(1) space-complexity per GME lock when the system contains a large number of GME locks as well as O(1) step-complexity when the system contains no conflicting requests
Coupled instabilities drive quasiperiodic order-disorder transitions in Faraday waves
We present an experimental study of quasiperiodic transitions between a
highly ordered square-lattice pattern and a disordered, defect-riddled state,
in a circular Faraday system. We show that the transition is driven initially
by a long-wave amplitude modulation instability, which excites the oscillatory
transition phase instability, leading to the formation of dislocations in the
Faraday lattice. The appearance of dislocations damps amplitude modulations,
which prevents further defects from being created and allows the system to
relax back to its ordered state. The process then repeats itself in a
quasiperiodic manner. Our experiments reveal a surprising coupling between two
distinct instabilities in the Faraday system, and suggest that such coupling
may provide a generic mechanism for quasiperiodicity in nonlinear driven
dissipative systems
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 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
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
Dynamical facilitation governs glassy dynamics in suspensions of colloidal ellipsoids
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,
201