1,173 research outputs found
Two-state shear diagrams for complex fluids in shear flow
The possible "phase diagrams'' for shear-induced phase transitions between two phases are collected. We consider shear-thickening and shear-thinning fluids, under conditions of both common strain rate and common stress in the two phases, and present the four fundamental shear stress vs. strain rate curves and discuss their concentration dependence. We outline how to construct more complicated phase diagrams, discuss in which class various experimental systems fall, and sketch how to reconstruct the phase diagrams from rheological measurements
Age-dependent transient shear banding in soft glasses
We study numerically the formation of long-lived transient shear bands during
shear startup within two models of soft glasses (a simple fluidity model and an
adapted `soft glassy rheology' model). The degree and duration of banding
depends strongly on the applied shear rate, and on sample age before shearing.
In both models the ultimate steady flow state is homogeneous at all shear
rates, consistent with the underlying constitutive curve being monotonic.
However, particularly in the SGR case, the transient bands can be extremely
long lived. The banding instability is neither `purely viscous' nor `purely
elastic' in origin, but is closely associated with stress overshoot in startup
flow.Comment: 4 pages, 3 figure
Diffusion and rheology in a model of glassy materials
We study self-diffusion within a simple hopping model for glassy materials.
(The model is Bouchaud's model of glasses [J.-P. Bouchaud, J. Physique I 2,
1705 (1992)], as extended to describe rheological properties [P. Sollich, F.
Lequeux, P. Hebraud and M.E. Cates, Phys. Rev. Lett. 78, 2020 (1997)].) We
investigate the breakdown, near the glass transition, of the (generalized)
Stokes-Einstein relation between self-diffusion of a tracer particle and the
(frequency-dependent) viscosity of the system as a whole. This stems from the
presence of a broad distribution of relaxation times of which different moments
control diffusion and rheology. We also investigate the effect of flow
(oscillatory shear) on self-diffusion and show that this causes a finite
diffusivity in the temperature regime below the glass transition (where this
was previously zero). At higher temperatures the diffusivity is enhanced by a
power law frequency dependence that also characterises the rheological
response. The relevance of these findings to soft glassy materials (foams,
emulsions etc.) as well as to conventional glass-forming liquids is discussed.Comment: 39 page (double spaced), 2 figure
Role of Metastable States in Phase Ordering Dynamics
We show that the rate of separation of two phases of different densities
(e.g. gas and solid) can be radically altered by the presence of a metastable
intermediate phase (e.g. liquid). Within a Cahn-Hilliard theory we study the
growth in one dimension of a solid droplet from a supersaturated gas. A moving
interface between solid and gas phases (say) can, for sufficient (transient)
supersaturation, unbind into two interfaces separated by a slab of metastable
liquid phase. We investigate the criteria for unbinding, and show that it may
strongly impede the growth of the solid phase.Comment: 4 pages, Latex, Revtex, epsf. Updated two reference
Tricritical behavior in dynamical phase transitions
We identify a new scenario for dynamical phase transitions associated with
time-integrated observables occurring in diffusive systems described by the
macroscopic fluctuation theory. It is characterized by the pairwise meeting of
first- and second-order bias-induced phase transition curves at two tricritical
points. We formulate a simple, general criterion for its appearance and derive
an exact Landau theory for the tricritical behavior. The scenario is
demonstrated in three examples: the simple symmetric exclusion process biased
by an activity-related structural observable; the Katz-Lebowitz-Spohn lattice
gas model biased by its current; and in an active lattice gas biased by its
entropy production.Comment: 21 pages, authors' accepted versio
Large time dynamics and aging of a polymer chain in a random potential
We study the out-of-equilibrium large time dynamics of a gaussian polymer
chain in a quenched random potential. The dynamics studied is a simple Langevin
dynamics commonly referred to as the Rouse model. The equations for the
two-time correlation and response function are derived within the gaussian
variational approximation. In order to implement this approximation faithfully,
we employ the supersymmetric representation of the Martin-Siggia-Rose dynamical
action. For a short ranged correlated random potential the equations are solved
analytically in the limit of large times using certain assumptions concerning
the asymptotic behavior. Two possible dynamical behaviors are identified
depending upon the time separation- a stationary regime and an aging regime. In
the stationary regime time translation invariance holds and so is the
fluctuation dissipation theorem. The aging regime which occurs for large time
separations of the two-time correlation functions is characterized by history
dependence and the breakdown of certain equilibrium relations. The large time
limit of the equations yields equations among the order parameters that are
similar to the equations obtained in the statics using replicas. In particular
the aging solution corresponds to the broken replica solution. But there is a
difference in one equation that leads to important consequences for the
solution. The stationary regime corresponds to the motion of the polymer inside
a local minimum of the random potential, whereas in the aging regime the
polymer hops between different minima. As a byproduct we also solve exactly the
dynamics of a chain in a random potential with quadratic correlations.Comment: 21 pages, RevTeX
Nonequilibrium dynamics of mixtures of active and passive colloidal particles
We develop a mesoscopic field theory for the collective nonequilibrium
dynamics of multicomponent mixtures of interacting active (i.e., motile) and
passive (i.e., nonmotile) colloidal particles with isometric shape in two
spatial dimensions. By a stability analysis of the field theory, we obtain
equations for the spinodal that describes the onset of a motility-induced
instability leading to cluster formation in such mixtures. The prediction for
the spinodal is found to be in good agreement with particle-resolved computer
simulations. Furthermore, we show that in active-passive mixtures the spinodal
instability can be of two different types. One type is associated with a
stationary bifurcation and occurs also in one-component active systems, whereas
the other type is associated with a Hopf bifurcation and can occur only in
active-passive mixtures. Remarkably, the Hopf bifurcation leads to moving
clusters. This explains recent results from simulations of active-passive
particle mixtures, where moving clusters and interfaces that are not seen in
the corresponding one-component systems have been observed.Comment: 17 pages, 3 figure
Active-passive mixtures with bulk loading: a minimal active engine in one dimension
We study a one-dimensional mixture of active (run-and-tumble) particles and
passive (Brownian) particles, with single-file constraint, in a sawtooth
potential. The active particles experience a ratchet effect: this generates a
current, which can push passive particles against an applied load. The
resulting system operates as an active engine. Using numerical simulations, we
analyse the efficiency of this engine, and we discuss how it can be optimised.
Efficient operation occurs when the active particles self-organise into teams,
which can push the passive ones against large loads by leveraging collective
behaviour. We discuss how the particle arrangement, conserved under the
single-file constraint, affects the engine efficiency. We also show that
relaxing this constraint still allows the engine to operate effectively.Comment: 21 pages, 17 figure
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