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## From Over-charging to Like-charge Attraction in the Weak Coupling Regime

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 11/07/2011

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Despite decades of intensive studies, the effective interactions between
strongly charged colloids still remain elusive. Here we show that a strongly
charged surface with a layer of condensed counter- ions behaves effectively as
a conductor, due to the mobile nature of the condensed ions. An external source
charge in its vicinity is therefore attracted towards the surface, due to the
image charge effect. This mechanism leads to correlational energies for
counter-ions condensed on two distinct surfaces, as well as for free ions in
the bulk. Generalizing Debye-Huckel theory and image charge methods, we
analytically calculate these correlation energies for the two-plates problem,
at the iso-electric point, where condensed counterions precisely balance the
bare surface charges. At this point, the effective interaction between two
plates is always attractive at small separation and repulsive at large
separation.; Comment: 5 pages, 3 eps figures

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## Polymers as compressible soft spheres

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/05/2012

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#Condensed Matter - Soft Condensed Matter#Condensed Matter - Mesoscale and Nanoscale Physics#Condensed Matter - Statistical Mechanics

We consider a coarse-grained model in which polymers under good-solvent
conditions are represented by soft spheres whose radii, which should be
identified with the polymer radii of gyrations, are allowed to fluctuate. The
corresponding pair potential depends on the sphere radii. This model is a
single-sphere version of the one proposed in Vettorel et al., Soft Matter 6,
2282 (2010), and it is sufficiently simple to allow us to determine all
potentials accurately from full-monomer simulations of two isolated polymers
(zero-density potentials). We find that in the dilute regime (which is the
expected validity range of single-sphere coarse-grained models based on
zero-density potentials) this model correctly reproduces the density dependence
of the radius of gyration. However, for the thermodynamics and the
intermolecular structure, the model is largely equivalent to the simpler one in
which the sphere radii are fixed to the average value of the radius of gyration
and radiiindependent potentials are used: for the thermodynamics there is no
advantage in considering a fluctuating sphere size.; Comment: 21 pages, 7 figures

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## Orientational Order in Liquids upon Condensation in Nanochannels: An Optical Birefringence Study on Rodlike and Disclike Molecules in Monolithic Mesoporous Silica

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 05/12/2010

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#Condensed Matter - Soft Condensed Matter#Condensed Matter - Mesoscale and Nanoscale Physics#Condensed Matter - Materials Science#Physics - Chemical Physics#Physics - Optics

We present high-resolution optical birefringence measurements upon sequential
filling of an array of parallel-aligned nanochannels (14~nm mean diameter) with
rod-like (acetonitrile) and disc-like (hexafluorobenzene) molecules. We will
demonstrate that such birefringence isotherms, when performed simultaneously
with optically isotropic and index-matched counterparts (neopentane and
hexafluoromethane), allow one to characterize the orientational state of the
confined liquids with a high accuracy as a function of pore filling. The pore
condensates are almost bulk-like, optically isotropic liquids. For both
anisotropic species we find, however, a weak orientational order (of a few
percent at maximum) upon film-condensation in the monolithic mesoporous
membrane. It occurs upon formation of the second and third adsorbed layer,
only, and vanishes gradually upon onset of capillary condensation. Presumably,
it originates in the breaking of the full rotational symmetry of the
interaction potential at the cylindrical, free liquid-vapor interface in the
film-condensed state rather than at the silica-liquid interface. This
conclusion is corroborated by comparisons of our experimental results with
molecular dynamics simulations reported in the literature.; Comment: 6 pages...

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## Phase behavior of hard spheres confined between parallel hard plates: Manipulation of colloidal crystal structures by confinement

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

55.56%

#Condensed Matter - Soft Condensed Matter#Condensed Matter - Materials Science#Condensed Matter - Statistical Mechanics

We study the phase behavior of hard spheres confined between two parallel
hard plates using extensive computer simulations. We determine the full
equilibrium phase diagram for arbitrary densities and plate separations from
one to five hard-sphere diameters using free energy calculations. We find a
first-order fluid-solid transition, which corresponds to either capillary
freezing or melting depending on the plate separation. The coexisting solid
phase consists of crystalline layers with either triangular or square symmetry.
Increasing the plate separation, we find a sequence of crystal structures from
n triangular to (n+1) square to (n+1) triangular, where n is the number of
crystal layers, in agreement with experiments on colloids. At high densities,
the transition between square to triangular phases are intervened by
intermediate structures, e.g., prism, buckled, and rhombic phases.; Comment: 9 pages, 4 figures. Accepted for publication in J. Phys.: Condens.
Matter

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## Correlation between crystalline order and vitrification in colloidal monolayers

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 08/12/2014

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#Condensed Matter - Soft Condensed Matter#Condensed Matter - Materials Science#Condensed Matter - Statistical Mechanics

We investigate experimentally the relationship between local structure and
dynamical arrest in a quasi-2d colloidal model system which approximates hard
discs. We introduce polydispersity to the system to suppress crystallisation.
Upon compression, the increase in structural relaxation time is accompanied by
the emergence of local hexagonal symmetry. Examining the dynamical
heterogeneity of the system, we identify three types of motion :
"zero-dimensional" corresponding to beta-relaxation, "one-dimensional" or
stringlike motion and "two-dimensional" motion. The dynamic heterogeneity is
correlated with the local order, that is to say locally hexagonal regions are
more likely to be dynamically slow. However we find that lengthscales
corresponding to dynamic heterogeneity and local structure do not appear to
scale together approaching the glass transition.; Comment: 13 papes, to appear in J. Phys.: Condens. Matter

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## Aging as dynamics in configuration space

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 07/05/1999

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55.56%

#Condensed Matter - Soft Condensed Matter#Condensed Matter - Disordered Systems and Neural Networks#Condensed Matter - Statistical Mechanics

The relaxation dynamics of many disordered systems, such as structural
glasses, proteins, granular materials or spin glasses, is not completely frozen
even at very low temperatures. This residual motion leads to a change of the
properties of the material, a process commonly called aging. Despite recent
advances in the theoretical description of such aging processes, the
microscopic mechanisms leading to the aging dynamics are still a matter of
dispute. In this Letter we investigate the aging dynamics of a simple glass
former by means of molecular dynamics computer simulation. Using the concept of
the inherent structure we give evidence that aging dynamics can be understood
as a decrease of the effective configurational temperature T of the system.
From our results we conclude that the equilibration process is faster when
the system is quenched to T_c, the critical T of mode-coupling theory, and that
thermodynamic concepts are useful to describe the out-of-equilibrium aging
process.; Comment: Latex 4 figures

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## Self-assembling DNA-caged particles: nanoblocks for hierarchical self-assembly

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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#Condensed Matter - Soft Condensed Matter#Condensed Matter - Materials Science#Condensed Matter - Statistical Mechanics

DNA is an ideal candidate to organize matter on the nanoscale, primarily due
to the specificity and complexity of DNA based interactions. Recent advances in
this direction include the self-assembly of colloidal crystals using DNA
grafted particles. In this article we theoretically study the self-assembly of
DNA-caged particles. These nanoblocks combine DNA grafted particles with more
complicated purely DNA based constructs. Geometrically the nanoblock is a
sphere (DNA grafted particle) inscribed inside a polyhedron (DNA cage). The
faces of the DNA cage are open, and the edges are made from double stranded
DNA. The cage vertices are modified DNA junctions. We calculate the
equilibriuim yield of self-assembled, tetrahedrally caged particles, and
discuss their stability with respect to alternative structures. The
experimental feasability of the method is discussed. To conclude we indicate
the usefulness of DNA-caged particles as nanoblocks in a hierarchical
self-assembly strategy.; Comment: v2: 21 pages, 8 figures; revised discussion in Sec. 2, replaced 2
figures, added new references

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## Nematic twist-bend phase with nanoscale modulation of molecular orientation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 03/09/2013

Relevância na Pesquisa

55.56%

#Condensed Matter - Soft Condensed Matter#Condensed Matter - Mesoscale and Nanoscale Physics#Condensed Matter - Materials Science#Physics - Chemical Physics

A state of matter in which molecules show a long-range orientational order
and no positional order is called a nematic liquid crystal. The best known and
most widely used (for example, in modern displays) is the uniaxial nematic,
with the rod-like molecules aligned along a single axis, called the director.
When the molecules are chiral, the director twists in space, drawing a
right-angle helicoid and remaining perpendicular to the helix axis; the
structure is called a chiral nematic. In this work, using transmission electron
and optical microscopy, we experimentally demonstrate a new nematic order,
formed by achiral molecules, in which the director follows an oblique helicoid,
maintaining a constant oblique angle with the helix axis and experiencing twist
and bend. The oblique helicoids have a nanoscale pitch. The new twist-bend
nematic represents a structural link between the uniaxial nematic (no tilt) and
a chiral nematic (helicoids with right-angle tilt).; Comment: 31 pages: 8 Figures and Supplementary Information with 3 Figures

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## Connecting short and long time dynamics in hard-sphere-like colloidal glasses

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 02/12/2014

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Glass-forming materials are characterized by an intermittent motion at the
microscopic scale. Particles spend most of their time rattling within the cages
formed by their neighbors, and seldom jump to a different cage. In molecular
glass formers the temperature dependence of the jump features, such as the
average caging time and jump length, characterizes the relaxation processes and
allows for a short-time prediction of the diffusivity. Here we experimentally
investigate the cage-jump motion of a two-dimensional hard-sphere-like
colloidal suspension, where the volume fraction is the relevant parameter
controlling the slowing down of the dynamics. We characterize the volume
fraction dependence of the cage-jump features and show that, as in molecular
systems, they allow for a short time prediction of the diffusivity.; Comment: 5 pages, 6 figures, Soft Matter 2015

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## Yielding dynamics of a Herschel-Bulkley fluid: a critical-like fluidization behaviour

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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The shear-induced fluidization of a carbopol microgel is investigated during
long start-up experiments using combined rheology and velocimetry in Couette
cells of varying gap widths and boundary conditions. As already described in
[Divoux et al., {\it Phys. Rev. Lett.}, 2010, {\bf 104}, 208301], we show that
the fluidization process of this simple yield stress fluid involves a transient
shear-banding regime whose duration $\tau_f$ decreases as a power law of the
applied shear rate $\gp$. Here we go one step further by an exhaustive
investigation of the influence of the shearing geometry through the gap width
$e$ and the boundary conditions. While slip conditions at the walls seem to
have a negligible influence on the fluidization time $\tau_f$, different
fluidization processes are observed depending on $\gp$ and $e$: the shear band
remains almost stationary for several hours at low shear rates or small gap
widths before strong fluctuations lead to a homogeneous flow, whereas at larger
values of $\gp$ or $e$, the transient shear band is seen to invade the whole
gap in a much smoother way. Still, the power-law behaviour appears as very
robust and hints to critical-like dynamics. To further discuss these results,
we propose (i) a qualitative scenario to explain the induction-like period that
precedes full fluidization and (ii) an analogy with critical phenomena that
naturally leads to the observed power laws if one assumes that the yield point
is the critical point of an underlying out-of-equilibrium phase transition.; Comment: 16 pages...

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## Capillary leveling of stepped films with inhomogeneous molecular mobility

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 14/06/2013

Relevância na Pesquisa

55.56%

#Condensed Matter - Soft Condensed Matter#Condensed Matter - Mesoscale and Nanoscale Physics#Condensed Matter - Materials Science#Physics - Chemical Physics#Physics - Fluid Dynamics

A homogeneous thin polymer film with a stepped height profile levels due to
the presence of Laplace pressure gradients. Here we report on studies of
polymeric samples with precisely controlled, spatially inhomogeneous molecular
weight distributions. The viscosity of a polymer melt strongly depends on the
chain length distribution; thus, we learn about thin-film hydrodynamics with
viscosity gradients. These gradients are achieved by stacking two films with
different molecular weights atop one another. After a sufficient time these
samples can be well described as having one dimensional viscosity gradients in
the plane of the film, with a uniform viscosity normal to the film. We develop
a hydrodynamic model that accurately predicts the shape of the experimentally
observed self-similar profiles. The model allows for the extraction of a
capillary velocity, the ratio of the surface tension and the viscosity, in the
system. The results are in excellent agreement with capillary velocity
measurements of uniform mono- and bi-disperse stepped films and are consistent
with bulk polymer rheology.; Comment: Accepted for publication in Soft Matter, Themed Issue on "The
Geometry and Topology of Soft Materials"

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## The Geometry of Soft Materials: A Primer

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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We present an overview of the differential geometry of curves and surfaces
using examples from soft matter as illustrations. The presentation requires a
background only in vector calculus and is otherwise self-contained.; Comment: 45 pages, RevTeX, 12 eps figures

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## A micromechanical model of collapsing quicksand

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 13/12/2010

Relevância na Pesquisa

55.56%

#Condensed Matter - Soft Condensed Matter#Condensed Matter - Materials Science#Condensed Matter - Statistical Mechanics#Physics - Computational Physics

The discrete element method constitutes a general class of modeling
techniques to simulate the microscopic behavior (i.e. at the particle scale) of
granular/soil materials. We present a contact dynamics method, accounting for
the cohesive nature of fine powders and soils. A modification of the model
adjusted to capture the essential physical processes underlying the dynamics of
generation and collapse of loose systems is able to simulate "quicksand"
behavior of a collapsing soil material, in particular of a specific type, which
we call "living quicksand". We investigate the penetration behavior of an
object for varying density of the material. We also investigate the dynamics of
the penetration process, by measuring the relation between the driving force
and the resulting velocity of the intruder, leading to a "power law" behavior
with exponent 1/2, i.e. a quadratic velocity dependence of the drag force on
the intruder.; Comment: 5 pages, 4 figures, accepted for granular matter

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## Non--Newtonian viscosity of interacting Brownian particles: comparison of theory and data

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 09/10/2002

Relevância na Pesquisa

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#Condensed Matter - Soft Condensed Matter#Condensed Matter - Disordered Systems and Neural Networks#Condensed Matter - Statistical Mechanics

A recent first-principles approach to the non-linear rheology of dense
colloidal suspensions is evaluated and compared to simulation results of
sheared systems close to their glass transitions. The predicted scenario of a
universal transition of the structural dynamics between yielding of glasses and
non-Newtonian (shear-thinning) fluid flow appears well obeyed, and calculations
within simplified models rationalize the data over variations in shear rate and
viscosity of up to 3 decades.; Comment: 6 pages, 2 figures; J. Phys. Condens. Matter to be published (Jan.
2003)

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## Dynamical heterogeneity in aging colloidal glasses of Laponite

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

55.56%

Glasses behave as solids due to their long relaxation time; however the
origin of this slow response remains a puzzle. Growing dynamic length scales
due to cooperative motion of particles are believed to be central to the
understanding of both the slow dynamics and the emergence of rigidity. Here, we
provide experimental evidence of a growing dynamical heterogeneity length scale
that increases with increasing waiting time in an aging colloidal glass of
Laponite. The signature of heterogeneity in the dynamics follows from dynamic
light scattering measurements in which we study both the rotational and
translational diffusion of the disk-shaped particles of Laponite in suspension.
These measurements are accompanied by simultaneous microrheology and
macroscopic rheology experiments. We find that rotational diffusion of
particles slows down at a faster rate than their translational motion. Such
decoupling of translational and orientational degrees of freedom finds its
origin in the dynamic heterogeneity since rotation and translation probe
different length scales in the sample. The macroscopic rheology experiments
show that the low frequency shear viscosity increases at a much faster rate
than both rotational and translational diffusive relaxation times.; Comment: 12 pages...

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## The nature of the glass and gel transitions in sticky spheres

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 18/09/2014

Relevância na Pesquisa

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Glasses and gels are the two dynamically arrested, disordered states of
matter. Despite their importance, their similarities and differences remain
elusive, especially at high density. We identify dynamical and structural
signatures which distinguish the gel and glass transitions in a colloidal model
system of hard and "sticky" spheres. Gelation is induced by crossing the
gas-liquid phase-separation line and the resulting rapid densification of the
colloid-rich phase leads to a sharp change in dynamics and local structure.
Thus, we find that gelation is first-order-like and can occur at much higher
densities than previously thought: far from being low-density networks, gels
have a clear "thermodynamic" definition which nevertheless leads to a
non-equilibrium state with a distinct local structure characteristic of a
rapidly quenched glass. In contrast, approaching the glass transition, the
dynamics slow continuously accompanied by the emergence of local five-fold
symmetric structure. Our findings provide a general thermodynamic, kinetic, and
structural basis upon which to distinguish gelation from vitrification.; Comment: 12 pages

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## Wet to dry crossover and a flow vortex-lattice in active nematics

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/05/2015

Relevância na Pesquisa

55.56%

#Condensed Matter - Soft Condensed Matter#Condensed Matter - Statistical Mechanics#Physics - Fluid Dynamics

Active systems, from bacterial suspensions to vibrated granular matter, are
continuously driven out of equilibrium by local injection of energy from their
constituent elements. The energy input leads to exotic behaviour such as
collective motion, pattern formation, topological defects and active
turbulence, but theories that link the different manifestations of activity
across systems and length scales are lacking. Here we unify two different
classes of active matter by using friction as a control parameter to
interpolate between wet active systems, whose behaviour is dominated by
hydrodynamics, and dry active matter where any flow is screened. At the wet-dry
crossover, we find a novel lattice of flow vortices interleaved with an ordered
network of topological defects which arises from the competition between
friction and viscous dissipation. Our results contribute to understanding the
physics of matter operating out-of-equilibrium, with its potential in the
design of active micro- and nano-machines.

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## Absence of `fragility' and mechanical response of jammed granular materials

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 08/02/2012

Relevância na Pesquisa

55.56%

We perform molecular dynamic (MD) simulations of frictional non-thermal
particles driven by an externally applied shear stress. After the system jams
following a transient flow, we probe its mechanical response in order to
clarify whether the resulting solid is 'fragile'. We find the system to respond
elastically and isotropically to small perturbations of the shear stress,
suggesting absence of fragility. These results are interpreted in terms of the
energy landscape of dissipative systems. For the same values of the control
parameters, we check the behaviour of the system during a stress cycle.
Increasing the maximum stress value, a crossover from a visco-elastic to a
plastic regime is observed.; Comment: 6 pages, 9 figures, accepted in Granular Matter on 01-02-2012

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## From cage-jump motion to macroscopic diffusion in supercooled liquids

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

55.56%

The evaluation of the long term stability of a material requires the
estimation of its long-time dynamics. For amorphous materials such as
structural glasses, it has proven difficult to predict the long-time dynamics
starting from static measurements. Here we consider how long one needs to
monitor the dynamics of a structural glass to predict its long--time features.
We present a detailed characterization of the statistical features of the
single-particle intermittent motion of structural glasses, and show that
single--particle jumps are the irreversible events leading to the relaxation of
the system. This allows to evaluate the diffusion constant on the time--scale
of the jump duration, which is small and temperature independent, well before
the system enters the diffusive regime. The prediction is obtained by analyzing
the particle trajectories via a parameter-free algorithm.; Comment: 6 pages, 5 figures, accepted for publication in Soft Matter (21 May
2014)

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## The effects of polydispersity and metastability on crystal growth kinetics

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

55.57%

#Condensed Matter - Soft Condensed Matter#Condensed Matter - Mesoscale and Nanoscale Physics#Condensed Matter - Statistical Mechanics

We investigate the effect of metastable gas-liquid (G-L) separation on
crystal growth in a system of either monodisperse or slightly size-polydisperse
square well particles, using a simulation setup that allows us to focus on the
growth of a single crystal. Our system parameters are such that, inside the
metastable G-L binodal, a macroscopic layer of the gas phase "coats" the
crystal as it grows, consistent with experiment and theoretical free energy
considerations. Crucially, the effect of this metastable G-L separation on the
crystal growth rate depends qualitatively on whether the system is
polydisperse. We measure reduced polydispersity and qualitatively different
local size ordering in the crystal relative to the fluid, proposing that the
required fractionation is dynamically facilitated by the gas layer. Our results
show that polydispersity and metastability, both ubiquitous in soft matter,
must be considered in tandem if their dynamical effects are to be understood.; Comment: Published in Soft Matter. DOI: 10.1039/C3SM27627A

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