20,502 research outputs found
The effective temperature
This review presents the effective temperature notion as defined from the
deviations from the equilibrium fluctuation-dissipation theorem in out of
equilibrium systems with slow dynamics. The thermodynamic meaning of this
quantity is discussed in detail. Analytic, numeric and experimental
measurements are surveyed. Open issues are mentioned.Comment: 58 page
Nonlinear rheological properties of dense colloidal dispersions close to a glass transition under steady shear
The nonlinear rheological properties of dense colloidal suspensions under
steady shear are discussed within a first principles approach. It starts from
the Smoluchowski equation of interacting Brownian particles in a given shear
flow, derives generalized Green-Kubo relations, which contain the transients
dynamics formally exactly, and closes the equations using mode coupling
approximations. Shear thinning of colloidal fluids and dynamical yielding of
colloidal glasses arise from a competition between a slowing down of structural
relaxation, because of particle interactions, and enhanced decorrelation of
fluctuations, caused by the shear advection of density fluctuations. The
integration through transients approach takes account of the dynamic
competition, translational invariance enters the concept of wavevector
advection, and the mode coupling approximation enables to quantitatively
explore the shear-induced suppression of particle caging and the resulting
speed-up of the structural relaxation. Extended comparisons with shear stress
data in the linear response and in the nonlinear regime measured in model
thermo-sensitive core-shell latices are discussed. Additionally, the single
particle motion under shear observed by confocal microscopy and in computer
simulations is reviewed and analysed theoretically.Comment: Review submited to special volume 'High Solid Dispersions' ed. M.
Cloitre, Vol. xx of 'Advances and Polymer Science' (Springer, Berlin, 2009);
some figures slightly cu
Phase coexistence in a monolayer of active particles induced by Marangoni flows
Thermally or chemically active colloids generate thermodynamic gradients in
the solution in which they are immersed and thereby induce hydrodynamic flows
that affect their dynamical evolution. Here we study a mean-field model for the
many-body dynamics of a monolayer of active particles located at a fluid-fluid
interface. In this case, the activity of the particles creates long-ranged
Marangoni flows due to the response of the interface, which compete with the
direct interaction between the particles. For the most interesting case of a
soft repulsion that models the electrostatic or magnetic interparticle
forces, we show that an "onion-like" density distribution will develop within
the monolayer. For a sufficiently large average density, two-dimensional phase
transitions (freezing from liquid to hexatic, and melting from solid to
hexatic) should be observable in a radially stratified structure. Furthermore,
the analysis allows us to conclude that, while the activity may be too weak to
allow direct detection of such induced Marangoni flows, it is relevant as a
collective effect in the emergence of the experimentally observable spatial
structure of phase coexistences noted above. Finally, the relevance of these
results for potential experimental realizations is critically discussed.Comment: 11 page
Cooperativity flows and Shear-Bandings: a statistical field theory approach
Cooperativity effects have been proposed to explain the non-local rheology in
the dynamics of soft jammed systems. Based on the analysis of the free-energy
model proposed by L. Bocquet, A. Colin \& A. Ajdari ({\em Phys. Rev. Lett.}
{\bf 103}, 036001 (2009)), we show that cooperativity effects resulting from
the non-local nature of the fluidity (inverse viscosity), are intimately
related to the emergence of shear-banding configurations. This connection
materializes through the onset of inhomogeneous compact solutions (compactons),
wherein the fluidity is confined to finite-support subregions of the flow and
strictly zero elsewhere. Compactons coexistence with regions of zero fluidity
("non-flowing vacuum") is shown to be stabilized by the presence of mechanical
noise, which ultimately shapes up the equilibrium distribution of the fluidity
field, the latter acting as an order parameter for the flow-noflow transitions
occurring in the material.Comment: 33 pages, 10 figure
Yielding dynamics of a Herschel-Bulkley fluid: a critical-like fluidization behaviour
The shear-induced fluidization of a carbopol microgel is investigated during
long start-up experiments using combined rheology and velocimetry in Couette
cells of varying gap widths and boundary conditions. As already described in
[Divoux et al., {\it Phys. Rev. Lett.}, 2010, {\bf 104}, 208301], we show that
the fluidization process of this simple yield stress fluid involves a transient
shear-banding regime whose duration decreases as a power law of the
applied shear rate \gp. Here we go one step further by an exhaustive
investigation of the influence of the shearing geometry through the gap width
and the boundary conditions. While slip conditions at the walls seem to
have a negligible influence on the fluidization time , different
fluidization processes are observed depending on \gp and : the shear band
remains almost stationary for several hours at low shear rates or small gap
widths before strong fluctuations lead to a homogeneous flow, whereas at larger
values of \gp or , the transient shear band is seen to invade the whole
gap in a much smoother way. Still, the power-law behaviour appears as very
robust and hints to critical-like dynamics. To further discuss these results,
we propose (i) a qualitative scenario to explain the induction-like period that
precedes full fluidization and (ii) an analogy with critical phenomena that
naturally leads to the observed power laws if one assumes that the yield point
is the critical point of an underlying out-of-equilibrium phase transition.Comment: 16 pages, 14+2 figures, published in Soft Matte
Flow-Induced Helical Coiling of Semiflexible Polymers in Structured Microchannels
The conformations of semiflexible (bio)polymers are studied in flow through
geometrically structured microchannels. Using mesoscale hydrodynamics
simulations, we show that the polymer undergoes a rod-to-helix transition as it
moves from the narrow high-velocity region into the wide low-velocity region of
the channel. The transient helix formation is the result of a non-equilibrium
and non-stationary buckling transition of the semiflexible polymer, which is
subjected to a compressive force originating from the fluid-velocity variation
in the channel. The helix properties depend on the diameter ratio of the
channel, the polymer bending rigidity, and the flow strength.Comment: Accepted in Phys. Rev. Let
Role of hydrodynamic flows in chemically driven droplet division
We study the hydrodynamics and shape changes of chemically active droplets.
In non-spherical droplets, surface tension generates hydrodynamic flows that
drive liquid droplets into a spherical shape. Here we show that spherical
droplets that are maintained away from thermodynamic equilibrium by chemical
reactions may not remain spherical but can undergo a shape instability which
can lead to spontaneous droplet division. In this case chemical activity acts
against surface tension and tension-induced hydrodynamic flows. By combining
low Reynolds-number hydrodynamics with phase separation dynamics and chemical
reaction kinetics we determine stability diagrams of spherical droplets as a
function of dimensionless viscosity and reaction parameters. We determine
concentration and flow fields inside and outside the droplets during shape
changes and division. Our work shows that hydrodynamic flows tends to stabilize
spherical shapes but that droplet division occurs for sufficiently strong
chemical driving, sufficiently large droplet viscosity or sufficiently small
surface tension. Active droplets could provide simple models for prebiotic
protocells that are able to proliferate. Our work captures the key
hydrodynamics of droplet division that could be observable in chemically active
colloidal droplets
Blood crystal: emergent order of red blood cells under wall-confined shear flow
Driven or active suspensions can display fascinating collective behavior,
where coherent motions or structures arise on a scale much larger than that of
the constituent particles. Here, we report experiments and numerical
simulations revealing that red blood cells (RBCs) assemble into regular
patterns in a confined shear flow. The order is of pure hydrodynamic and
inertialess origin, and emerges from a subtle interplay between (i)
hydrodynamic repulsion by the bounding walls which drives deformable cells
towards the channel mid-plane and (ii) intercellular hydrodynamic interactions
which can be attractive or repulsive depending on cell-cell separation. Various
crystal-like structures arise depending on RBC concentration and confinement.
Hardened RBCs in experiments and rigid particles in simulations remain
disordered under the same conditions where deformable RBCs form regular
patterns, highlighting the intimate link between particle deformability and the
emergence of order. The difference in structuring ability of healthy
(deformable) and diseased (stiff) RBCs creates a flow signature potentially
exploitable for diagnosis of blood pathologies
Out of equilibrium dynamics of classical and quantum complex systems
Equilibrium is a rather ideal situation, the exception rather than the rule
in Nature. Whenever the external or internal parameters of a physical system
are varied its subsequent relaxation to equilibrium may be either impossible or
take very long times. From the point of view of fundamental physics no generic
principle such as the ones of thermodynamics allows us to fully understand
their behaviour. The alternative is to treat each case separately. It is
illusionary to attempt to give, at least at this stage, a complete description
of all non-equilibrium situations. Still, one can try to identify and
characterise some concrete but still general features of a class of out of
equilibrium problems - yet to be identified - and search for a unified
description of these. In this report I briefly describe the behaviour and
theory of a set of non-equilibrium systems and I try to highlight common
features and some general laws that have emerged in recent years.Comment: 36 pages, to be published in Compte Rendus de l'Academie de Sciences,
T. Giamarchi e
Thermostat for non-equilibrium multiparticle collision dynamics simulations
Multiparticle collision dynamics (MPC), a particle-based mesoscale simulation
technique for com- plex fluid, is widely employed in non-equilibrium
simulations of soft matter systems. To maintain a defined thermodynamic state,
thermalization of the fluid is often required for certain MPC variants. We
investigate the influence of three thermostats on the non-equilibrium
properties of a MPC fluid under shear or in Poiseuille flow. In all cases, the
local velocities are scaled by a factor, which is either determined via a local
simple scaling approach (LSS), a Monte Carlo-like procedure (MCS), or by the
Maxwell-Boltzmann distribution of kinetic energy (MBS). We find that the
various scal- ing schemes leave the flow profile unchanged and maintain the
local temperature well. The fluid viscosities extracted from the various
simulations are in close agreement. Moreover, the numerically determined
viscosities are in remarkably good agreement with the respective theoretically
predicted values. At equilibrium, the calculation of the dynamic structure
factor reveals that the MBS method closely resembles an isothermal ensemble,
whereas the MCS procedure exhibits signatures of an adi- abatic system at
larger collision-time steps. Since the velocity distribution of the LSS
approach is non-Gaussian, we recommend to apply the MBS thermostat, which has
been shown to produce the correct velocity distribution even under
non-equilibrium conditions.Comment: 12 pages, 5 figures in Phys. Rev. E, 201
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