56 research outputs found
Density profiles of a colloidal liquid at a wall under shear flow
Using a dynamical density functional theory we analyze the density profile of
a colloidal liquid near a wall under shear flow. Due to the symmetries of the
system considered, the naive application of dynamical density functional theory
does not lead to a shear induced modification of the equilibrium density
profile, which would be expected on physical grounds. By introducing a
physically motivated dynamic mean field correction we incorporate the missing
shear induced interparticle forces into the theory. We find that the shear flow
tends to enhance the oscillations in the density profile of hard-spheres at a
hard-wall and, at sufficiently high shear rates, induces a nonequilibrium
transition to a steady state characterized by planes of particles parallel to
the wall. Under gravity, we find that the center-of-mass of the density
distribution increases with shear rate, i.e., shear increases the potential
energy of the particles
On the interplay between sedimentation and phase separation phenomena in two-dimensional colloidal fluids
Colloidal particles that are confined to an interface effectively form a
two-dimensional fluid. We examine the dynamics of such colloids when they are
subject to a constant external force, which drives them in a particular
direction over the surface. Such a situation occurs, for example, for colloidal
particles that have settled to the bottom of their container, when the
container is tilted at an angle, so that they `sediment' to the lower edge of
the surface. We focus in particular on the case when there are attractive
forces between the colloids which causes them to phase separate into regions of
high density and low density and we study the influence of this phase
separation on the sedimentation process. We model the colloids as Brownian
particles and use both Brownian dynamics computer simulations and dynamical
density functional theory (DDFT) to obtain the time evolution of the ensemble
average one-body density profiles of the colloids. We consider situations where
the external potential varies only in one direction so that the ensemble
average density profiles vary only in this direction. We solve the DDFT in
one-dimension, by assuming that the density profile only varies in one
direction. However, we also solve the DDFT in two-dimensions, allowing the
fluid density profile to vary in both the - and -directions. We find that
in certain situations the two-dimensional DDFT is clearly superior to its
one-dimensional counterpart when compared with the simulations and we discuss
this issue.Comment: 17 pages, 10 figures, submitted to Molecular Physic
Understanding the dynamics of biological colloids to elucidate cataract formation towards the development of methodology for its early diagnosis
The eye lens is the most characteristic example of mammalian tissues
exhibiting complex colloidal behaviour. In this paper we briefly describe how
dynamics in colloidal suspensions can help addressing selected aspects of lens
cataract which is ultimately related to the protein self-assembly under
pathological conditions. Results from dynamic light scattering of eye lens
homogenates over a wide protein concentration were analyzed and the various
relaxation modes were identified in terms of collective and self-diffusion
processes. Using this information as an input, the complex relaxation pattern
of the intact lens nucleus was rationalized. The model of cold cataract - a
phase separation effect of the lens cytoplasm with cooling - was used to
simulate lens cataract at in vitro conditions in an effort to determine the
parameters of the correlation functions that can be used as reliable indicators
of the cataract onset. The applicability of dynamic light scattering as a
non-invasive, early-diagnostic tool for ocular diseases is also demonstrated in
the light of the findings of the present paper.Comment: Slightly different version from the published one 10 pages, 2 figure
Many-particle Brownian and Langevin Dynamics Simulations with the Brownmove package
<p>Abstract</p> <p>Background</p> <p>Brownian Dynamics (BD) is a coarse-grained implicit-solvent simulation method that is routinely used to investigate binary protein association dynamics, but due to its efficiency in handling large simulation volumes and particle numbers it is well suited to also describe many-protein scenarios as they often occur in biological cells.</p> <p>Results</p> <p>Here we introduce our "brownmove" simulation package which was designed to handle many-particle problems with varying particle numbers and allows for a very flexible definition of rigid and flexible protein and polymer models. Both a Brownian and a Langevin dynamics (LD) propagation scheme can be used and hydrodynamic interactions are treated efficiently with our recently introduced TEA-HI ansatz [Geyer, Winter, JCP 130 (2009) 114905]. With simulations of constrained polymers and flexible models of spherical proteins we demonstrate that it is crucial to include hydrodynamics when multi-bead models are used in BD or LD simulations. Only then both the translational and the rotational diffusion coefficients and the timescales of the internal dynamics can be reproduced correctly. In the third example project we show how constant density boundary conditions [Geyer et al, JCP 120 (2004) 4573] can be used to set up a non-equilibrium simulation of diffusional transport across an array of fixed obstacles. Finally, we demonstrate how the agglomeration dynamics of multiple particles with attractive patches can be analysed conveniently with the help of a dynamic interaction network.</p> <p>Conclusions</p> <p>Combining BD and LD propagation, fast hydrodynamics, a flexible protein model, and interfaces for "open" simulation settings, our freely available "brownmove" simulation package constitutes a new platform for coarse-grained many-particle simulations of biologically relevant diffusion and transport processes.</p
Phenomenology and physical origin of shear-localization and shear-banding in complex fluids
We review and compare the phenomenological aspects and physical origin of
shear-localization and shear-banding in various material types, namely
emulsions, suspensions, colloids, granular materials and micellar systems. It
appears that shear-banding, which must be distinguished from the simple effect
of coexisting static-flowing regions in yield stress fluids, occurs in the form
of a progressive evolution of the local viscosity towards two significantly
different values in two adjoining regions of the fluids in which the stress
takes slightly different values. This suggests that from a global point of view
shear-banding in these systems has a common physical origin: two physical
phenomena (for example, in colloids, destructuration due to flow and
restructuration due to aging) are in competition and, depending on the flow
conditions, one of them becomes dominant and makes the system evolve in a
specific direction.Comment: The original publication is available at http://www.springerlink.co
Recent experimental probes of shear banding
Recent experimental techniques used to investigate shear banding are
reviewed. After recalling the rheological signature of shear-banded flows, we
summarize the various tools for measuring locally the microstructure and the
velocity field under shear. Local velocity measurements using dynamic light
scattering and ultrasound are emphasized. A few results are extracted from
current works to illustrate open questions and directions for future research.Comment: Review paper, 23 pages, 11 figures, 204 reference
Microstructure of a near-critical colloidal dispersion under stationary shear flow
Microstructural order of near-critical suspensions of colloid-polymer mixtures under stationary shear flow is investigated by means of small-angle light scattering. The experiments reveal a significant, unexpected shear-induced distortion in directions perpendicular to the flow direction, the more so on closer approach to the gas-liquid critical point. Starting from the N-particle Smoluchowski equation, we derive an explicit expression for the shear-rate dependence of the static structure factor. The present theory improves on a previous treatment, by taking the shear-induced distortions of short-ranged correlations into account, which are indeed shown to be important on close approach to the critical point. The experiments are in quantitative accordance with this extended theory, for all wavevectors, shear rates, and distances from the gas-liquid critical point.status: publishe
Fluid-fluid phase separation in colloid-polymer mixtures studied with small angle light scattering and light microscopy
Crystallization kinetics of colloidal spheres under stationary shear flow
A systematic experimental study of dispersions of charged colloidal spheres is presented on the effect of steady shear flow on nucleation and crystal growth rates. In addition, the nonequilibrium phase diagram as it relates to the melting line is measured. Shear flow is found to strongly affect induction times, crystal growth rates, and the location of the melting line. The main findings are that (1) the crystal growth rate for a given concentration exhibits a maximum as a function of the shear rate; (2) contrary to the monotonic increase in the growth rate with increasing concentration in the absence of flow, a maximum of the crystal growth rate as a function of concentration is observed for sheared systems; and (3) the induction time for a given concentration exhibits a maximum as a function of the shear rate. These findings are partly explained on a qualitative level.status: publishe
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