711 research outputs found
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 non-equilibrium phase diagram as far as
the melting line is concerned 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 (i) the crystal growth rate for a given
concentration exhibits a maximum as a function of the shear rate, (ii) contrary
to the monotonous increase of 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 (iii) the induction time for
a given concentration exhibits a maximum as a function of the shear rate. These
findings will be partly explained on a qualitative level.Comment: 17 pages, 10 figures, accepted in Langmui
Efficient simulation of non-crossing fibers and chains in a hydrodynamic solvent
An efficient simulation method is presented for Brownian fiber suspensions,
which includes both uncrossability of the fibers and hydrodynamic interactions
between the fibers mediated by a mesoscopic solvent. To conserve hydrodynamics,
collisions between the fibers are treated such that momentum and energy are
conserved locally. The choice of simulation parameters is rationalised on the
basis of dimensionless numbers expressing the relative strength of different
physical processes. The method is applied to suspensions of semiflexible fibers
with a contour length equal to the persistence length, and a mesh size to
contour length ratio ranging from 0.055 to 0.32. For such fibers the effects of
hydrodynamic interactions are observable, but relatively small. The
non-crossing constraint, on the other hand, is very important and leads to
hindered displacements of the fibers, with an effective tube diameter in
agreement with recent theoretical predictions. The simulation technique opens
the way to study the effect of viscous effects and hydrodynamic interactions in
microrheology experiments where the response of an actively driven probe bead
in a fiber suspension is measured.Comment: 12 pages, 2 tables, 5 figure
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
Note: Scale-free center-of-mass displacement correlations in polymer films without topological constraints and momentum conservation
We present here computational work on the center-of-mass displacements in
thin polymer films of finite width without topological constraints and without
momentum conservation obtained using a well-known lattice Monte Carlo algorithm
with chain lengths ranging up to N=8192. Computing directly the center-of-mass
displacement correlation function C_N(t) allows to make manifest the existence
of scale-free colored forces acting on a reference chain. As suggested by the
scaling arguments put forward in a recent work on three-dimensional melts, we
obtain a negative algebraic decay C_N(t) \sim -1/(Nt) for times t << T_N with
T_N being the chain relaxation time. This implies a logarithmic correction to
the related center-of-mass mean square-displacement h_N(t) as has been checked
directly
Kinetic pathways of the Nematic-Isotropic phase transition as studied by confocal microscopy on rod-like viruses
We investigate the kinetics of phase separation for a mixture of rodlike
viruses (fd) and polymer (dextran), which effectively constitutes a system of
attractive rods. This dispersion is quenched from a flow-induced fully nematic
state into the region where the nematic and the isotropic phase coexist. We
show experimental evidence that the kinetic pathway depends on the overall
concentration. When the quench is made at high concentrations, the system is
meta-stable and we observe typical nucleation-and-growth. For quenches at low
concentration the system is unstable and the system undergoes a spinodal
decomposition. At intermediate concentrations we see the transition between
both demixing processes, where we locate the spinodal point.Comment: 11 pages, 6 figures, accepted in J. Phys.: Condens. Matter as
symposium paper for the 6th Liquid Matter Conference in Utrech
Dynamic Monte Carlo Simulations of Anisotropic Colloids
We put forward a simple procedure for extracting dynamical information from
Monte Carlo simulations, by appropriate matching of the short-time diffusion
tensor with its infinite-dilution limit counterpart, which is supposed to be
known. This approach --discarding hydrodynamics interactions-- first allows us
to improve the efficiency of previous Dynamic Monte Carlo algorithms for
spherical Brownian particles. In a second step, we address the case of
anisotropic colloids with orientational degrees of freedom. As an illustration,
we present a detailed study of the dynamics of thin platelets, with emphasis on
long-time diffusion and orientational correlations.Comment: 12 pages, 9 figure
Substrate concentration dependence of the diffusion-controlled steady-state rate constant
The Smoluchowski approach to diffusion-controlled reactions is generalized to
interacting substrate particles by including the osmotic pressure and
hydrodynamic interactions of the nonideal particles in the Smoluchoswki
equation within a local-density approximation. By solving the strictly
linearized equation for the time-independent case with absorbing boundary
conditions, we present an analytic expression for the diffusion-limited
steady-state rate constant for small substrate concentrations in terms of an
effective second virial coefficient B_2*. Comparisons to Brownian dynamics
simulations excluding HI show excellent agreement up to bulk number densities
of B_2* rho_0 < 0.4 for hard sphere and repulsive Yukawa-like interactions
between the substrates. Our study provides an alternative way to determine the
second virial coefficient of interacting macromolecules experimentally by
measuring their steady-state rate constant in diffusion-controlled reactions at
low densities.Comment: 7 pages, 3 figure
Dense colloidal suspensions under time-dependent shear
We consider the nonlinear rheology of dense colloidal suspensions under a
time-dependent simple shear flow. Starting from the Smoluchowski equation for
interacting Brownian particles advected by shearing (ignoring fluctuations in
fluid velocity) we develop a formalism which enables the calculation of
time-dependent, far-from-equilibrium averages. Taking shear-stress as an
example we derive exactly a generalized Green-Kubo relation, and an equation of
motion for the transient density correlator, involving a three-time memory
function. Mode coupling approximations give a closed constitutive equation
yielding the time-dependent stress for arbitrary shear rate history. We solve
this equation numerically for the special case of a hard sphere glass subject
to step-strain.Comment: 4 page
Hydrodynamic induced deformation and orientation of a microscopic elastic filament
We describe simulations of a microscopic elastic filament immersed in a fluid
and subject to a uniform external force. Our method accounts for the
hydrodynamic coupling between the flow generated by the filament and the
friction force it experiences. While models that neglect this coupling predict
a drift in a straight configuration, our findings are very different. Notably,
a force with a component perpendicular to the filament axis induces bending and
perpendicular alignment. Moreover, with increasing force we observe four shape
regimes, ranging from slight distortion to a state of tumbling motion that
lacks a steady state. We also identify the appearance of marginally stable
structures. Both the instability of these shapes and the observed alignment can
be explained by the combined action of induced bending and non-local
hydrodynamic interactions. Most of these effects should be experimentally
relevant for stiff micro-filaments, such as microtubules.Comment: three figures. To appear in Phys Rev Let
Stochastic Interactions of Two Brownian Hard Spheres in the Presence of Depletants
A quantitative analysis is presented for the stochastic interactions of a
pair of Brownian hard spheres in non-adsorbing polymer solutions. The hard
spheres are hypothetically trapped by optical tweezers and allowed for random
motion near the trapped positions. The investigation focuses on the long-time
correlated Brownian motion. The mobility tensor altered by the polymer
depletion effect is computed by the boundary integral method, and the
corresponding random displacement is determined by the fluctuation-dissipation
theorem. From our computations it follows that the presence of depletion layers
around the hard spheres has a significant effect on the hydrodynamic
interactions and particle dynamics as compared to pure solvent and pure polymer
solution (no depletion) cases. The probability distribution functions of random
walks of the two interacting hard spheres that are trapped clearly shifts due
to the polymer depletion effect. The results show that the reduction of the
viscosity in the depletion layers around the spheres and the entropic force due
to the overlapping of depletion zones have a significant influence on the
correlated Brownian interactions.Comment: 30 pages, 9 figures, 1 appendix, 40 formulas inside the text, 5
formulas in appendi
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