1,126 research outputs found
Critical scaling of jammed system after quench of temperature
Critical behavior of soft repulsive particles after quench of temperature
near the jamming trasition is numerically investigated. It is found that the
plateau of the mean square displacement of tracer particles and the pressure
satisfy critical scaling laws. The critical density for the jamming transition
depends on the protocol to prepare the system, while the values of the critical
exponents which are consistent with the prediction of a phenomenology are
independent of the protocol.Comment: 7 pages, 9 figures, to appear in Phys. Rev.
Crystallization of hard-sphere glasses
We study by molecular dynamics the interplay between arrest and
crystallization in hard spheres. For state points in the plane of volume
fraction () and polydispersity (), we delineate states that spontaneously crystallize from those that do
not. For noncrystallizing (or precrystallization) samples we find
isodiffusivity lines consistent with an ideal glass transition at , independent of . Despite this, for , crystallization
occurs at . This happens on time scales for which the system is
aging, and a diffusive regime in the mean square displacement is not reached;
by those criteria, the system is a glass. Hence, contrary to a widespread
assumption in the colloid literature, the occurrence of spontaneous
crystallization within a bulk amorphous state does not prove that this state
was an ergodic fluid rather than a glass.Comment: 4 pages, 3 figure
Properties of cage rearrangements observed near the colloidal glass transition
We use confocal microscopy to study the motions of particles in concentrated
colloidal systems. Near the glass transition, diffusive motion is inhibited, as
particles spend time trapped in transient ``cages'' formed by neighboring
particles. We measure the cage sizes and lifetimes, which respectively shrink
and grow as the glass transition approaches. Cage rearrangements are more
prevalent in regions with lower local concentrations and higher disorder.
Neighboring rearranging particles typically move in parallel directions,
although a nontrivial fraction move in anti-parallel directions, usually from
pairs of particles with initial separations corresponding to the local maxima
and minima of the pair correlation function , respectively.Comment: 5 pages, 4 figures; text & figures revised in v
Equilibrium phase behavior of polydisperse hard spheres
We calculate the phase behavior of hard spheres with size polydispersity,
using accurate free energy expressions for the fluid and solid phases. Cloud
and shadow curves, which determine the onset of phase coexistence, are found
exactly by the moment free energy method, but we also compute the complete
phase diagram, taking full account of fractionation effects. In contrast to
earlier, simplified treatments we find no point of equal concentration between
fluid and solid or re-entrant melting at higher densities. Rather, the fluid
cloud curve continues to the largest polydispersity that we study (14%); from
the equilibrium phase behavior a terminal polydispersity can thus only be
defined for the solid, where we find it to be around 7%. At sufficiently large
polydispersity, fractionation into several solid phases can occur, consistent
with previous approximate calculations; we find in addition that coexistence of
several solids with a fluid phase is also possible
Dynamics of hard-sphere suspension using Dynamic Light Scattering and X-Ray Photon Correlation Spectroscopy: dynamics and scaling of the Intermediate Scattering Function
Intermediate Scattering Functions (ISF's) are measured for colloidal hard
sphere systems using both Dynamic Light Scattering (DLS) and X-ray Photon
Correlation Spectroscopy (XPCS). We compare the techniques, and discuss the
advantages and disadvantages of each. Both techniques agree in the overlapping
range of scattering vectors. We investigate the scaling behaviour found by
Segre and Pusey [1] but challenged by Lurio et al. [2]. We observe a scaling
behaviour over several decades in time but not in the long time regime.
Moreover, we do not observe long time diffusive regimes at scattering vectors
away from the peak of the structure factor and so question the existence of a
long time diffusion coefficients at these scattering vectors.Comment: 21 pages, 11 figure
Glasses in hard spheres with short-range attraction
We report a detailed experimental study of the structure and dynamics of
glassy states in hard spheres with short-range attraction. The system is a
suspension of nearly-hard-sphere colloidal particles and non-adsorbing linear
polymer which induces a depletion attraction between the particles. Observation
of crystallization reveals a re-entrant glass transition. Static light
scattering shows a continuous change in the static structure factors upon
increasing attraction. Dynamic light scattering results, which cover 11 orders
of magnitude in time, are consistent with the existence of two distinct kinds
of glasses, those dominated by inter-particle repulsion and caging, and those
dominated by attraction. Samples close to the `A3 point' predicted by mode
coupling theory for such systems show very slow, logarithmic dynamics.Comment: 22 pages, 18 figure
Critical behaviors of sheared frictionless granular materials near jamming transition
Critical behaviors of sheared dense and frictionless granular materials in
the vicinity of the jamming transition are numerically investigated. From the
extensive molecular dynamics simulation, we verify the validity of the scaling
theory near the jamming transition proposed by Otsuki and Hayakawa (Prog.
Theor. Phys., 121, 647 (2009)). We also clarify the critical behaviors of the
shear viscosity and the pair correlation function based on both a phenomenology
and the simulation.Comment: 13pages, 26 figure
Dynamic regimes of hydrodynamically coupled self-propelling particles
We analyze the collective dynamics of self-propelling particles (spps) which
move at small Reynolds numbers including the hydrodynamic coupling to the
suspending solvent through numerical simulations. The velocity distribution
functions show marked deviations from Gaussian behavior at short times, and the
mean-square displacement at long times shows a transition from diffusive to
ballistic motion for appropriate driving mechanism at low concentrations. We
discuss the structures the spps form at long times and how they correlate to
their dynamic behavior.Comment: 7 pages, 4 figure
The short-time self-diffusion coefficient of a sphere in a suspension of rigid rods
The short--time self diffusion coefficient of a sphere in a suspension of
rigid rods is calculated in first order in the rod volume fraction. For low rod
concentrations the correction to the Einstein diffusion constant of the sphere
is a linear function of the rod volume fraction with the slope proportional to
the equilibrium averaged mobility diminution trace of the sphere interacting
with a single freely translating and rotating rod. The two--body hydrodynamic
interactions are calculated using the so--called bead model in which the rod is
replaced by a stiff linear chain of touching spheres. The interactions between
spheres are calculated numerically using the multipole method. Also an
analytical expression for the diffusion coefficient as a function of the rod
aspect ratio is derived in the limit of very long rods. We show that in this
limit the correction to the Einstein diffusion constant does not depend on the
size of the tracer sphere. The higher order corrections depending on the
applied model are computed numerically. An approximate expression is provided,
valid for a wide range of aspect ratios.Comment: 11 pages, 6 figure
Phase Separation in Charge-Stabilized Colloidal Suspensions: Influence of Nonlinear Screening
The phase behavior of charge-stabilized colloidal suspensions is modeled by a
combination of response theory for electrostatic interparticle interactions and
variational theory for free energies. Integrating out degrees of freedom of the
microions (counterions, salt ions), the macroion-microion mixture is mapped
onto a one-component system governed by effective macroion interactions. Linear
response of microions to the electrostatic potential of the macroions results
in a screened-Coulomb (Yukawa) effective pair potential and a one-body volume
energy, while nonlinear response modifies the effective interactions [A. R.
Denton, \PR E {\bf 70}, 031404 (2004)]. The volume energy and effective pair
potential are taken as input to a variational free energy, based on
thermodynamic perturbation theory. For both linear and first-order nonlinear
effective interactions, a coexistence analysis applied to aqueous suspensions
of highly charged macroions and monovalent microions yields bulk separation of
macroion-rich and macroion-poor phases below a critical salt concentration, in
qualitative agreement with predictions of related linearized theories [R. van
Roij, M. Dijkstra, and J.-P. Hansen, \PR E {\bf 59}, 2010 (1999); P. B. Warren,
\JCP {\bf 112}, 4683 (2000)]. It is concluded that nonlinear screening can
modify phase behavior but does not necessarily suppress bulk phase separation
of deionized suspensions.Comment: 14 pages of text + 9 figure
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