73 research outputs found
Nonequilibrium equation of state in suspensions of active colloids
Active colloids constitute a novel class of materials composed of
colloidal-scale particles locally converting chemical energy into motility,
mimicking micro-organisms. Evolving far from equilibrium, these systems display
structural organizations and dynamical properties distinct from thermalized
colloidal assemblies. Harvesting the potential of this new class of systems
requires the development of a conceptual framework to describe these
intrinsically nonequilibrium systems. We use sedimentation experiments to probe
the nonequilibrium equation of state of a bidimensional assembly of active
Janus microspheres, and conduct computer simulations of a model of
self-propelled hard disks. Self-propulsion profoundly affects the equation of
state, but these changes can be rationalized using equilibrium concepts. We
show that active colloids behave, in the dilute limit, as an ideal gas with an
activity-dependent effective temperature. At finite density, increasing the
activity is similar to increasing adhesion between equilibrium particles. We
quantify this effective adhesion and obtain a unique scaling law relating
activity and effective adhesion in both experiments and simulations. Our
results provide a new and efficient way to understand the emergence of novel
phases of matter in active colloidal suspensions.Comment: 8 pages, 4 figs; to be published in Phys. Rev.
Sedimentation of active colloidal suspensions
In this paper, we investigate experimentally the non-equilibrium steady state
of an active colloidal suspension under gravity field. The active particles are
made of chemically powered colloids, showing self propulsion in the presence of
an added fuel, here hydrogen peroxide. The active suspension is studied in a
dedicated microfluidic device, made of permeable gel microstructures. Both the
microdynamics of individual colloids and the global stationary state of the
suspension under gravity - density profiles, number fluctuations - are measured
with optical microscopy. This allows to connect the sedimentation length to the
individual self-propelled dynamics, suggesting that in the present dilute
regime the active colloids behave as 'hot' particles. Our work is a first step
in the experimental exploration of the out-of-equilibrium properties of
artificial active systems.Comment: 4 pages, 4 figure
Ion specificity and anomalous electrokinetic effects in hydrophobic nanochannels
We demonstrate with computer simulations that anomalous electrokinetic
effects, such as ion specificity and non-zero zeta potentials for uncharged
surfaces, are generic features of electro-osmotic flow in hydrophobic channels.
This behavior is due to the stronger attraction of larger ions to the
``vapour--liquid-like'' interface induced by a hydrophobic surface. An
analytical model involving a modified Poisson--Boltzmann description for the
ion density distributions is proposed, which allows the anomalous flow profiles
to be predicted quantitatively. This description incorporates as a crucial
component an ion-size-dependent hydrophobic solvation energy. These results
provide an effective framework for predicting specific ion effects, with
important implications for the modeling of biological problems
Active glass: ergodicity breaking dramatically affects response to self-propulsion
We study experimentally the response of a dense sediment of Brownian
particles to self-propulsion. We observe that the ergodic supercooled liquid
relaxation is monotonically enhanced by activity. By contrast the nonergodic
glass shows an order of magnitude slowdown at low activities with respect to
passive case, followed by fluidization at higher activities. Our results
contrast with theoretical predictions of the ergodic approach to glass
transition summing up to a shift of the glass line. We propose that
nonmonotonicity is due to competing effects of activity: (i) extra energy that
helps breaking cages (ii) directionality that hinders cage exploration. We call
it "Deadlock from the Emergence of Active Directionality" (DEAD). It suggests
further theoretical works should include thermal motion.Comment: 5 pages, 3 figures + supplementary materials (3 pages, 5 figures
Aging or DEAD: origin of the non-monotonic response to weak self-propulsion in active glasses
Among amorphous states, glass is defined by relaxation times longer than the
observation time. This nonergodic nature makes the understanding of glassy
systems an involved topic, with complex aging effects or responses to further
out-of-equilibrium external drivings. In this respect active glasses made of
self-propelled particles have recently emerged as stimulating systems which
broadens and challenges our current understanding of glasses by considering
novel internal out-of-equilibrium degrees of freedom. In previous experimental
studies we have shown that in the ergodicity broken phase, the dynamics of
dense passive particles first slows down as particles are made slightly active,
before speeding up at larger activity. Here, we show that this nonmonotonic
behavior also emerges in simulations of soft active Brownian particles and
explore its cause. We refute that the Deadlock by Emergence of Active
Directionality (DEAD) model we proposed earlier describes our data. However, we
demonstrate that the nonmonotonic response is due to activity enhanced aging,
and thus confirm the link with ergodicity breaking. Beyond self-propelled
systems, our results suggest that aging in active glasses is not fully
understood
Self-propulsion of symmetric chemically active particles: Point-source model and experiments on camphor disks
International audienceSolid undeformable particles surrounded by a liquid medium or interface may propel themselves by altering their local environment. Such nonmechanical swimming is at work in autophoretic swimmers, whose self-generated field gradient induces a slip velocity on their surface, and in interfacial swimmers, which exploit unbalance in surface tension. In both classes of systems, swimmers with intrinsic asymmetry have received the most attention but self-propulsion is also possible for particles that are perfectly isotropic. The underlying symmetry-breaking instability has been established theoretically for autophoretic systems but has yet to be observed experimentally for solid particles. For interfacial swimmers, several experimental works point to such a mechanism, but its understanding has remained incomplete. The goal of this work is to fill this gap. Building on an earlier proposal, we first develop a point-source model that may be applied generically to interfacial or phoretic swimmers. Using this approximate but unifying picture, we show that they operate in very different regimes and obtain analytical predictions for the propulsion velocity and its dependence on swimmer size and asymmetry. Next, we present experiments on interfacial camphor disks showing that they indeed self-propel in an advection-dominated regime where intrinsic asymmetry is irrelevant and that the swimming velocity increases sublinearly with size. Finally, we discuss the merits and limitations of the point-source model in light of the experiments and point out its broader relevance
Chaotic mixing in effective compressible flows
International audienceWe study numerically joint mixing of salt and colloids by chaotic advection and how salt inhomogeneities accelerate or delay colloid mixing by inducing a velocity drift V dp between colloids and fluid particles as proposed in recent experiments [J. Deseigne et al., Soft Matter 10, 4795 (2014)]. We demonstrate that because the drift velocity is no longer divergence free, small variations to the total velocity field drastically affect the evolution of colloid variance Ï ^2 = â ^2. A consequence is that mixing strongly depends on the mutual coherence between colloid and salt concentration fields, the short time evolution of scalar variance being governed by a new variance production term P = â /2 when scalar gradients are not developed yet so that dissipation is weak. Depending on initial conditions, mixing is then delayed or enhanced, and it is possible to find examples for which the two regimes (fast mixing followed by slow mixing) are observed consecutively when the variance source term reverses its sign. This is indeed the case for localized patches modeled as Gaussian concentration profiles
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