17 research outputs found
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
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
Experimental signatures of a nonequilibrium phase transition governing the yielding of a soft glass
We present direct experimental signatures of a nonequilibrium phase transition associated with the yield point of a prototypical soft solid-a binary colloidal glass. By simultaneously quantifying single-particle dynamics and bulk mechanical response, we identified the threshold for the onset of irreversibility with the yield strain. We extracted the relaxation time from the transient behavior of the loss modulus and found that it diverges in the vicinity of the yield strain. This critical slowing down is accompanied by a growing correlation length associated with the size of regions of high Debye-Waller factor, which are precursors to yield events in glasses. Our results affirm that the paradigm of nonequilibrium critical phenomena is instrumental in achieving a holistic understanding of yielding in soft solids
Grain growth and grain boundary dynamics in colloidal polycrystals
Grain boundary dynamics and grain growth play a pivotal role in the fabrication of functional polycrystalline materials. However, not much is known about the delicate interplay between various microscopic processes that drive grain boundary motion which eventually culminates in the desired grain morphology. Colloidal systems are ideally suited to bridge the gap between the microscopic and macroscopic processes underlying grain growth, since their dynamics can be followed in real space and real time with single-particle resolution. The present review aims at highlighting contributions from colloid experiments that have led to a holistic understanding of grain growth in polycrystalline materials
Localized Excitations and the Morphology of Cooperatively Rearranging Regions in a Colloidal Glass-Forming Liquid
We develop a scheme based on a real space microscopic analysis of particle dynamics to ascertain the relevance of dynamical facilitation as a mechanism of structural relaxation in glass-forming liquids. By analyzing the spatial organization of localized excitations within clusters of mobile particles in a colloidal glass former and examining their partitioning into shell-like and corelike regions, we establish the existence of a crossover from a facilitation-dominated regime at low area fractions to a collective activated hopping-dominated one close to the glass transition. This crossover occurs in the vicinity of the area fraction at which the peak of the mobility transfer function exhibits a maximum and the morphology of cooperatively rearranging regions changes from stringlike to a compact form. Collectively, our findings suggest that dynamical facilitation is dominated by collective hopping close to the glass transition, thereby constituting a crucial step towards identifying the correct theoretical scenario for glass formation