715 research outputs found
Inertial effects in three dimensional spinodal decomposition of a symmetric binary fluid mixture: A lattice Boltzmann study
The late-stage demixing following spinodal decomposition of a
three-dimensional symmetric binary fluid mixture is studied numerically, using
a thermodynamicaly consistent lattice Boltzmann method. We combine results from
simulations with different numerical parameters to obtain an unprecendented
range of length and time scales when expressed in reduced physical units. Using
eight large (256^3) runs, the resulting composite graph of reduced domain size
l against reduced time t covers 1 < l < 10^5, 10 < t < 10^8. Our data is
consistent with the dynamical scaling hypothesis, that l(t) is a universal
scaling curve. We give the first detailed statistical analysis of fluid motion,
rather than just domain evolution, in simulations of this kind, and introduce
scaling plots for several quantities derived from the fluid velocity and
velocity gradient fields.Comment: 49 pages, latex, J. Fluid Mech. style, 48 embedded eps figs plus 6
colour jpegs for Fig 10 on p.2
Binary fluids under steady shear in three dimensions
We simulate by lattice Boltzmann the steady shearing of a binary fluid
mixture with full hydrodynamics in three dimensions. Contrary to some
theoretical scenarios, a dynamical steady state is attained with finite
correlation lengths in all three spatial directions. Using large simulations we
obtain at moderately high Reynolds numbers apparent scaling expon ents
comparable to those found by us previously in 2D. However, in 3D there may be a
crossover to different behavior at low Reynolds number: accessing this regime
requires even larger computational resource than used here.Comment: 4 pages, 3 figure
Tensorial Constitutive Models for Disordered Foams, Dense Emulsions, and other Soft Nonergodic Materials
In recent years, the paradigm of `soft glassy matter' has been used to
describe diverse nonergodic materials exhibiting strong local disorder and slow
mesoscopic rearrangement. As so far formulated, however, the resulting `soft
glassy rheology' (SGR) model treats the shear stress in isolation, effectively
`scalarizing' the stress and strain rate tensors. Here we offer generalizations
of the SGR model that combine its nontrivial aging and yield properties with a
tensorial structure that can be specifically adapted, for example, to the
description of fluid film assemblies or disordered foams.Comment: 18 pages, 4 figure
Velocity profiles in shear-banding wormlike micelles
Using Dynamic Light Scattering in heterodyne mode, we measure velocity
profiles in a much studied system of wormlike micelles (CPCl/NaSal) known to
exhibit both shear-banding and stress plateau behavior. Our data provide
evidence for the simplest shear-banding scenario, according to which the
effective viscosity drop in the system is due to the nucleation and growth of a
highly sheared band in the gap, whose thickness linearly increases with the
imposed shear rate. We discuss various details of the velocity profiles in all
the regions of the flow curve and emphasize on the complex, non-Newtonian
nature of the flow in the highly sheared band.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let
Rheology of Lamellar Liquid Crystals in Two and Three Dimensions: A Simulation Study
We present large scale computer simulations of the nonlinear bulk rheology of
lamellar phases (smectic liquid crystals) at moderate to large values of the
shear rate (Peclet numbers 10-100), in both two and three dimensions. In two
dimensions we find that modest shear rates align the system and stabilise an
almost regular lamellar phase, but high shear rates induce the nucleation and
proliferation of defects, which in steady state is balanced by the annihilation
of defects of opposite sign. The critical shear rate at onset of this second
regime is controlled by thermodynamic and kinetic parameters; we offer a
scaling analysis that relates the critical shear rate to a critical "capillary
number" involving those variables. Within the defect proliferation regime, the
defects may be partially annealed by slowly decreasing the applied shear rate;
this causes marked memory effects, and history-dependent rheology. Simulations
in three dimensions show instead shear-induced ordering even at the highest
shear rates studied here. This suggests that the critical shear rate shifts
markedly upward on increasing dimensionality. This may in part reflect the
reduced constraints on defect motion, allowing them to find and annihilate each
other more easily. Residual edge defects in the 3D aligned state mostly point
along the flow velocity, an orientation impossible in two dimensions.Comment: 18 pages, 12 figure
Fluctuating lattice Boltzmann
The lattice Boltzmann algorithm efficiently simulates the Navier Stokes
equation of isothermal fluid flow, but ignores thermal fluctuations of the
fluid, important in mesoscopic flows. We show how to adapt the algorithm to
include noise, satisfying a fluctuation-dissipation theorem (FDT) directly at
lattice level: this gives correct fluctuations for mass and momentum densities,
and for stresses, at all wavevectors . Unlike previous work, which recovers
FDT only as , our algorithm offers full statistical mechanical
consistency in mesoscale simulations of, e.g., fluctuating colloidal
hydrodynamics.Comment: 7 pages, 3 figures, to appear in Europhysics Letter
Coexistence and Phase Separation in Sheared Complex Fluids
We demonstrate how to construct dynamic phase diagrams for complex fluids
that undergo transitions under flow, in which the conserved composition
variable and the broken-symmetry order parameter (nematic, smectic,
crystalline, etc.) are coupled to shear rate. Our construction relies on a
selection criterion, the existence of a steady interface connecting two stable
homogeneous states. We use the (generalized) Doi model of lyotropic nematic
liquid crystals as a model system, but the method can be easily applied to
other systems, provided non-local effects are included.Comment: 4 pages REVTEX, 5 figures using epsf macros. To appear in Physical
Review E (Rapid Communications
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