39 research outputs found
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
Stochastic model for nucleosome sliding in the presence of DNA ligands
Heat-induced mobility of nucleosomes along DNA is an experimentally
well-studied phenomenon. A recent experiment shows that the repositioning is
modified in the presence of minor-groove binding DNA ligands. We present here a
stochastic three-state model for the diffusion of a nucleosome along DNA in the
presence of such ligands. It allows us to describe the dynamics and the steady
state of such a motion analytically. The analytical results are in excellent
agreement with numerical simulations of this stochastic process.With this
model, we study the response of a nucleosome to an external force and how it is
affected by the presence of ligands.Comment: 10 pages, 8 figures, submitted to Eur. Phys. J.
Long-Range Ordering of Vibrated Polar Disks
Vibrated polar disks have been used experimentally to investigate collective
motion of driven particles, where fully-ordered asymptotic regimes could not be
reached. Here we present a model reproducing quantitatively the single, binary
and collective properties of this granular system. Using system sizes not
accessible in the laboratory, we show in silico that true long-range order is
possible in the experimental system. Exploring the model's parameter space, we
find a phase diagram qualitatively different from that of dilute or point-like
particle systems.Comment: 5 pages, 4 figure
Traffic Instabilities in Self-Organized Pedestrian Crowds
In human crowds as well as in many animal societies, local interactions among
individuals often give rise to self-organized collective organizations that
offer functional benefits to the group. For instance, flows of pedestrians
moving in opposite directions spontaneously segregate into lanes of uniform
walking directions. This phenomenon is often referred to as a smart collective
pattern, as it increases the traffic efficiency with no need of external
control. However, the functional benefits of this emergent organization have
never been experimentally measured, and the underlying behavioral mechanisms
are poorly understood. In this work, we have studied this phenomenon under
controlled laboratory conditions. We found that the traffic segregation
exhibits structural instabilities characterized by the alternation of organized
and disorganized states, where the lifetime of well-organized clusters of
pedestrians follow a stretched exponential relaxation process. Further analysis
show that the inter-pedestrian variability of comfortable walking speeds is a
key variable at the origin of the observed traffic perturbations. We show that
the collective benefit of the emerging pattern is maximized when all
pedestrians walk at the average speed of the group. In practice, however, local
interactions between slow- and fast-walking pedestrians trigger global
breakdowns of organization, which reduce the collective and the individual
payoff provided by the traffic segregation. This work is a step ahead toward
the understanding of traffic self-organization in crowds, which turns out to be
modulated by complex behavioral mechanisms that do not always maximize the
group's benefits. The quantitative understanding of crowd behaviors opens the
way for designing bottom-up management strategies bound to promote the
emergence of efficient collective behaviors in crowds.Comment: Article published in PLoS Computational biology. Freely available
here:
http://www.ploscompbiol.org/article/info%3Adoi%2F10.1371%2Fjournal.pcbi.100244
Collective Motion and Phase Transitions of Symmetric Camphor Boats
The motion of several self-propelled boats in a narrow channel displays
spontaneous pattern formation and kinetic phase transitions. In contrast with
previous studies on self-propelled particles, this model does not require
stochastic fluctuations and it is experimentally accessible. By varying the
viscosity in the system, it is possible to form either a stationary state,
correlated or uncorrelated oscillations, or unidirectional flow. Here, we
describe and analyze these self organized patterns and their transitions.Comment: 6 pages, 6 figure
Pressure is not a state function for generic active fluids
Pressure is the mechanical force per unit area that a confined system exerts
on its container. In thermal equilibrium, it depends only on bulk properties
(density, temperature, etc.) through an equation of state. Here we show that in
a wide class of active systems the pressure depends on the precise interactions
between the active particles and the confining walls. In general, therefore,
active fluids have no equation of state, their mechanical pressures exhibit
anomalous properties that defy the familiar thermodynamic reasoning that holds
in equilibrium. The pressure remains a function of state, however, in some
specific and well-studied active models that tacitly restrict the character of
the particle-wall and/or particle-particle interactions.Comment: 8 pages + 9 SI pages, Nature Physics (2015
Active Brownian Particles. From Individual to Collective Stochastic Dynamics
We review theoretical models of individual motility as well as collective
dynamics and pattern formation of active particles. We focus on simple models
of active dynamics with a particular emphasis on nonlinear and stochastic
dynamics of such self-propelled entities in the framework of statistical
mechanics. Examples of such active units in complex physico-chemical and
biological systems are chemically powered nano-rods, localized patterns in
reaction-diffusion system, motile cells or macroscopic animals. Based on the
description of individual motion of point-like active particles by stochastic
differential equations, we discuss different velocity-dependent friction
functions, the impact of various types of fluctuations and calculate
characteristic observables such as stationary velocity distributions or
diffusion coefficients. Finally, we consider not only the free and confined
individual active dynamics but also different types of interaction between
active particles. The resulting collective dynamical behavior of large
assemblies and aggregates of active units is discussed and an overview over
some recent results on spatiotemporal pattern formation in such systems is
given.Comment: 161 pages, Review, Eur Phys J Special-Topics, accepte
Topological sound in active-liquid metamaterials
Liquids composed of self-propelled particles have been experimentally
realized using molecular, colloidal, or macroscopic constituents. These active
liquids can flow spontaneously even in the absence of an external drive. Unlike
spontaneous active flow, the propagation of density waves in confined active
liquids is not well explored. Here, we exploit a mapping between density waves
on top of a chiral flow and electrons in a synthetic gauge field to lay out
design principles for artificial structures termed topological active
metamaterials. We design metamaterials that break time-reversal symmetry using
lattices composed of annular channels filled with a spontaneously flowing
active liquid. Such active metamaterials support topologically protected sound
modes that propagate unidirectionally, without backscattering, along either
sample edges or domain walls and despite overdamped particle dynamics. Our work
illustrates how parity-symmetry breaking in metamaterial structure combined
with microscopic irreversibility of active matter leads to novel
functionalities that cannot be achieved using only passive materials
Coupling the Leidenfrost effect and elastic deformations to power sustained bouncing
The Leidenfrost effect occurs when an object near a hot surface vaporizes
rapidly enough to lift itself up and hover. Although well-understood for
liquids and stiff sublimable solids, nothing is known about the effect with
materials whose stiffness lies between these extremes. Here we introduce a new
phenomenon that occurs with vaporizable soft solids: the elastic Leidenfrost
effect. By dropping hydrogel spheres onto hot surfaces we find that, rather
than hovering, they energetically bounce several times their diameter for
minutes at a time. With high-speed video during a single impact, we uncover
high-frequency microscopic gap dynamics at the sphere-substrate interface. We
show how these otherwise-hidden agitations constitute work cycles that harvest
mechanical energy from the vapour and sustain the bouncing. Our findings
unleash a powerful and widely applicable strategy for injecting mechanical
energy into soft materials, with relevance to fields ranging from soft robotics
and metamaterials to microfluidics and active matter