1,385 research outputs found
The microscopic pathway to crystallization in supercooled liquids
Despite its fundamental and technological importance, a microscopic
understanding of the crystallization process is still elusive. By computer
simulations of the hard-sphere model we reveal the mechanism by which thermal
fluctuations drive the transition from the supercooled liquid state to the
crystal state. In particular we show that fluctuations in bond orientational
order trigger the nucleation process, contrary to the common belief that the
transition is initiated by density fluctuations. Moreover, the analysis of bond
orientational fluctuations shows that these not only act as seeds of the
nucleation process, but also i) determine the particular polymorph which is to
be nucleated from them and ii) at high density favour the formation of fivefold
structures which can frustrate the formation of crystals. These results can
shed new light on our understanding of the relationship between crystallization
and vitrification.Comment: to appear in "Scientific Reports
Understanding water's anomalies with locally favored structures
Water is a complex structured liquid of hydrogen-bonded molecules that
displays a surprising array of unusual properties, also known as water
anomalies, the most famous being the density maximum at about C. The
origin of these anomalies is still a matter of debate, and so far a
quantitative description of water's phase behavior starting from the molecular
arrangements is still missing. Here we provide a simple physical description
from microscopic data obtained through computer simulations. We introduce a
novel structural order parameter, which quantifies the degree of translational
order of the second shell, and show that this parameter alone, which measures
the amount of locally favored structures, accurately characterizes the state of
water. A two-state modeling of these microscopic structures is used to describe
the behavior of liquid water over a wide region of the phase diagram, correctly
identifying the density and compressibility anomalies, and being compatible
with the existence of a second critical point in the deeply supercooled region.
Furthermore, we reveal that locally favored structures in water not only have
translational order in the second shell, but also contain five-membered rings
of hydrogen-bonded molecules. This suggests their mixed character: the former
helps crystallization, whereas the latter causes frustration against
crystallization.Comment: 10 pages, 5 figure
A novel particle tracking method with individual particle size measurement and its application to ordering in glassy hard sphere colloids
Particle tracking is a key to single-particle-level confocal microscopy
observation of colloidal suspensions, emulsions, and granular matter. The
conventional tracking method has not been able to provide accurate information
on the size of individual particle. Here we propose a novel method to localise
spherical particles of arbitrary relative sizes from either 2D or 3D (confocal)
images either in dilute or crowded environment. Moreover this method allows us
to estimate the size of each particle reliably. We use this method to analyse
local bond orientational ordering in a supercooled polydisperse colloidal
suspension as well as the heterogeneous crystallisation induced by a substrate.
For the former, we reveal non-trivial couplings of crystal-like bond
orientational order and local icosahedral order with the spatial distribution
of particle sizes: Crystal-like order tends to form in regions where very small
particles are depleted and the slightly smaller size of the central particle
stabilizes icosahedral order. For the latter, on the other hand, we found that
very small particles are expelled from crystals and accumulated on the growth
front of crystals. We emphasize that such information has not been accessible
by conventional tracking methods
Roles of energy dissipation in a liquid-solid transition of out-of-equilibrium systems
Self-organization of active matter as well as driven granular matter in
non-equilibrium dynamical states has attracted considerable attention not only
from the fundamental and application viewpoints but also as a model to
understand the occurrence of such phenomena in nature. These systems share
common features originating from their intrinsically out-of-equilibrium nature.
It remains elusive how energy dissipation affects the state selection in such
non-equilibrium states. As a simple model system, we consider a non-equilibrium
stationary state maintained by continuous energy input, relevant to industrial
processing of granular materials by vibration and/or flow. More specifically,
we experimentally study roles of dissipation in self-organization of a driven
granular particle monolayer. We find that the introduction of strong
inelasticity entirely changes the nature of the liquid-solid transition from
two-step (nearly) continuous transitions (liquid-hexatic-solid) to a strongly
discontinuous first-order-like one (liquid-solid), where the two phases with
different effective temperatures can coexist, unlike thermal systems, under a
balance between energy input and dissipation. Our finding indicates a pivotal
role of energy dissipation and suggests a novel principle in the
self-organization of systems far from equilibrium. A similar principle may
apply to active matter, which is another important class of out-of-equilibrium
systems. On noting that interaction forces in active matter, and particularly
in living systems, are often non-conservative and dissipative, our finding may
also shed new light on the state selection in these systems.Comment: 17 pages, 11 figure
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