16 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.
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
Recoil experiments determine the eigenmodes of viscoelastic fluids
We experimentally investigate the recoil dynamics of a colloidal probe
particle after shearing it with constant velocity through a viscoelastic fluid.
The recoil displays two distinct timescales which are in excellent agreement
with a microscopic model built on a particle being linked to two bath particles
by harmonic springs. This model yields analytical expressions which reproduce
all experimental protocols, including additional waiting periods before
particle release. Notably, two sets of timescales appear, corresponding to
reciprocal and nonreciprocal eigenmodes of the model.Comment: 5 pages, 4 figure
Notes on factorization algebras, factorization homology and applications
These notes are an expanded version of two series of lectures given at the
winter school in mathematical physics at les Houches and at the Vietnamese
Institute for Mathematical Sciences. They are an introduction to factorization
algebras, factorization homology and some of their applications, notably for
studying -algebras. We give an account of homology theory for manifolds
(and spaces), which give invariant of manifolds but also invariant of
-algebras. We particularly emphasize the point of view of factorization
algebras (a structure originating from quantum field theory) which plays, with
respect to homology theory for manifolds, the role of sheaves with respect to
singular cohomology. We mention some applications to the study of mapping
spaces and study several examples, including some over stratified spaces.Comment: 122 pages. A few examples adde
Sedimentation experiments for active colloids
La matière colloïdale active composée d'assemblées de particules active, convertissant localement un carburant chimique en solution pour créer du mouvement, forme un système expérimental intéressant pour étudier les effets collectifs. Dans ce travail nous utilisons des expériences de sédimentation afin de sonder l'équation d'état hors équilibre d'une assemblée bidimensionnelle de colloïdes Janus actifs. Notre approche consiste à utiliser des concepts de physique à l'équilibre afin de décrire les comportements hors équilibre du système. Nous montrons qu'à basse densité ces colloïdes se comportent comme un gaz idéal, avec une température effective qui dépend de l'activité du système. Lorsqu'on augmente la densité du système, l'activité est responsable de l'apparition d'une adhésion effective entre les particules. Nous avons pu quantifier cette adhésion et obtenir une relation entre adhésion effective et activité du système
Solute effects in confined freezing
10 pages, 2 figures, 23 referencesThe presence of liquid water in frozen media impacts the strength of soils, the growth of frost heave, plant life and microbial activities, or the durability of infrastructures in cold regions. If the effect of confinement on freezing is well known, water is never pure and solutes depressing the freezing point are naturally found. Moreover, the combination of confinement and solute is poorly understood. We imaged the freezing dynamics of water in a model porous medium with various salt (KCl) concentrations. We showed that the freezing front, initially heterogeneous due to confinement, drives salt enrichment in the remaining liquid, further depressing its freezing point. Confinement and solute have a synergistic effect that results in much larger mushy layers and greater freezing point depression. These results should help understand the distribution of water in frozen porous media, solute precipitation and redistribution in soils, and cryo-tolerance of construction materials and organisms
Solute strongly impacts freezing under confinement
International audienceThe presence of liquid water in frozen media impacts the strength of soils, the growth of frost heave, plant life and microbial activities, or the durability of infrastructures in cold regions. If the effect of confinement alone on freezing is well known, water is never pure and solutes depressing the freezing point are naturally found. However, the combination of confinement and solute is poorly understood. Here, we study in situ the freezing of water in a model porous medium made of densely packed particles with various salt (KCl) concentrations. We demonstrate a synergistic effect of solute with confinement: the freezing front, initially heterogeneous due to confinement, drives solute enrichment in the remaining liquid, further depressing its freezing point. This increases the local freezing point depression and results in much larger mushy layers where ice and liquid water coexist. We compare our experimental freezing profile with theory and estimate the local solute concentration to increase by more than one order of magnitude through the freezing process. These results imply that even low solute concentrations may have important effects on the distribution of water in frozen porous media and should help explain the variety of freezing patterns observed experimentally. This may be critical for cryo-tolerance of construction materials and organisms and will help understanding solute precipitation and redistribution in soils
Two step micro-rheological behavior in a viscoelastic fluid
We perform micro-rheological experiments with a colloidal bead driven through
a viscoelastic worm-like micellar fluid and observe two distinctive shear
thinning regimes, each of them displaying a Newtonian-like plateau. The shear
thinning behavior at larger velocities is in qualitative agreement with
macroscopic rheological experiments. The second process, observed at
Weissenberg numbers as small as a few percent, appears to have no analog in
macro rheological findings. A simple model introduced earlier captures the
observed behavior, and implies that the two shear thinning processes correspond
to two different length scales in the fluid. This model also reproduces
oscillations which have been observed in this system previously. While the
system under macro-shear seems to be near equilibrium for shear rates in the
regime of the intermediate Newtonian-like plateau, the one under micro-shear is
thus still far from it. The analysis suggests the existence of a length scale
of a few micrometres, the nature of which remains elusive.Comment: 7 pages, 5 figure