965 research outputs found
Jamming, two-fluid behaviour and 'self-filtration' in concentrated particulate suspensions
We study the flow of model experimental hard sphere colloidal suspensions at
high volume fraction driven through a constriction by a pressure
gradient. Above a particle-size dependent limit , direct microscopic
observations demonstrate jamming and unjamming--conversion of fluid to solid
and vice versa--during flow. We show that such a jamming flow produces a
reduction in colloid concentration downstream of the constriction.
We propose that this `self-filtration' effect is the consequence of a
combination of jamming of the particulate part of the system and continuing
flow of the liquid part, i.e. the solvent, through the pores of the jammed
solid. Thus we link the concept of jamming in colloidal and granular media with
a 'two-fluid'-like picture of the flow of concentrated suspensions. Results are
also discussed in the light of Osborne Reynolds' original experiments on
dilation in granular materials.Comment: 4 pages, 3 figure
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
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
Density functional theory for the freezing of soft-core fluids
We present a simple density functional theory for the solid phases of systems
of particles interacting via soft-core potentials. In particular, we apply the
theory to particles interacting via repulsive point Yukawa and Gaussian pair
potentials. We find qualitative agreement with the established phase diagrams
for these systems. The theory is able to account for the bcc-fcc solid
transitions of both systems and the re-entrant melting that the Gaussian system
exhibits.Comment: 7 pages, 4 figure
Phase diagram of softly repulsive systems: The Gaussian and inverse-power-law potentials
We redraw, using state-of-the-art methods for free-energy calculations, the
phase diagrams of two reference models for the liquid state: the Gaussian and
inverse-power-law repulsive potentials. Notwithstanding the different behavior
of the two potentials for vanishing interparticle distances, their
thermodynamic properties are similar in a range of densities and temperatures,
being ruled by the competition between the body-centered-cubic (BCC) and
face-centered-cubic (FCC) crystalline structures and the fluid phase. We
confirm the existence of a reentrant BCC phase in the phase diagram of the
Gaussian-core model, just above the triple point. We also trace the BCC-FCC
coexistence line of the inverse-power-law model as a function of the power
exponent and relate the common features in the phase diagrams of such
systems to the softness degree of the interaction.Comment: 22 pages, 8 figure
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.
Hard Spheres: Crystallization and Glass Formation
Motivated by old experiments on colloidal suspensions, we report molecular
dynamics simulations of assemblies of hard spheres, addressing crystallization
and glass formation. The simulations cover wide ranges of polydispersity s
(standard deviation of the particle size distribution divided by its mean) and
particle concentration. No crystallization is observed for s > 0.07. For 0.02 <
s < 0.07, we find that increasing the polydispersity at a given concentration
slows down crystal nucleation. The main effect here is that polydispersity
reduces the supersaturation since it tends to stabilise the fluid but to
destabilise the crystal. At a given polydispersity (< 0.07) we find three
regimes of nucleation: standard nucleation and growth at concentrations in and
slightly above the coexistence region; "spinodal nucleation", where the free
energy barrier to nucleation appears to be negligible, at intermediate
concentrations; and, at the highest concentrations, a new mechanism, still to
be fully understood, which only requires small re-arrangement of the particle
positions. The cross-over between the second and third regimes occurs at a
concentration, around 58% by volume, where the colloid experiments show a
marked change in the nature of the crystals formed and the particle dynamics
indicate an "ideal" glass transition
Fluid-fluid phase separation in hard spheres with a bimodal size distribution
The effect of polydispersity on the phase behaviour of hard spheres is
examined using a moment projection method. It is found that the
Boublik-Mansoori-Carnahan-Starling-Leland equation of state shows a spinodal
instability for a bimodal distribution if the large spheres are sufficiently
polydisperse, and if there is sufficient disparity in mean size between the
small and large spheres. The spinodal instability direction points to the
appearance of a very dense phase of large spheres.Comment: 7 pages, 3 figures, moderately REVISED following referees' comments
(original was 4 pages, 3 postscript figures
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
Mixtures of Charged Colloid and Neutral Polymer: Influence of Electrostatic Interactions on Demixing and Interfacial Tension
The equilibrium phase behavior of a binary mixture of charged colloids and
neutral, non-adsorbing polymers is studied within free-volume theory. A model
mixture of charged hard-sphere macroions and ideal, coarse-grained,
effective-sphere polymers is mapped first onto a binary hard-sphere mixture
with non-additive diameters and then onto an effective Asakura-Oosawa model [S.
Asakura and F. Oosawa, J. Chem. Phys. 22, 1255 (1954)]. The effective model is
defined by a single dimensionless parameter -- the ratio of the polymer
diameter to the effective colloid diameter. For high salt-to-counterion
concentration ratios, a free-volume approximation for the free energy is used
to compute the fluid phase diagram, which describes demixing into colloid-rich
(liquid) and colloid-poor (vapor) phases. Increasing the range of electrostatic
interactions shifts the demixing binodal toward higher polymer concentration,
stabilizing the mixture. The enhanced stability is attributed to a weakening of
polymer depletion-induced attraction between electrostatically repelling
macroions. Comparison with predictions of density-functional theory reveals a
corresponding increase in the liquid-vapor interfacial tension. The predicted
trends in phase stability are consistent with observed behavior of
protein-polysaccharide mixtures in food colloids.Comment: 16 pages, 5 figure
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