50 research outputs found
Self-organized hydrodynamics with density-dependent velocity
Acknowledgments. This work has been supported by the Agence Nationale pour la Recherche (ANR) under grant ‘MOTIMO’ (ANR-11-MONU-009-01), by the Engineering and Physical Sciences Research Council (EPSRC) under grant ref. EP/M006883/1, and by the National Science Foundation (NSF) under grant RNMS 11-07444 (KI-Net). P. D. is on leave from CNRS, Institut de Math ́ematiques, Toulouse, France. He acknowledges support from the Royal Society and the Wolfson foundation through a Royal Society Wolfson Research Merit Award. H. Y. wishes to acknowledge the hospitality of the Department of Mathematics, Imperial College London, where this research was conducted. P. D. and H. Y. wish to thank F. Plourabou ́e (IMFT, Toulouse, France) for enlighting discussions.Peer reviewedPublisher PD
Active Jamming: Self-propelled soft particles at high density
We study numerically the phases and dynamics of a dense collection of
self-propelled particles with soft repulsive interactions in two dimensions.
The model is motivated by recent in vitro experiments on confluent monolayers
of migratory epithelial and endothelial cells. The phase diagram exhibits a
liquid phase with giant number fluctuations at low packing fraction and high
self-propulsion speed and a jammed phase at high packing fraction and low
self-propulsion speed. The dynamics of the jammed phase is controlled by the
low frequency modes of the jammed packing.Comment: 4 pages, 4 figure
The Statistical Physics of Athermal Materials
At the core of equilibrium statistical mechanics lies the notion of
statistical ensembles: a collection of microstates, each occurring with a given
a priori probability that depends only on a few macroscopic parameters such as
temperature, pressure, volume, and energy. In this review article, we discuss
recent advances in establishing statistical ensembles for athermal materials.
The broad class of granular and particulate materials is immune from the
effects of thermal fluctuations because the constituents are macroscopic. In
addition, interactions between grains are frictional and dissipative, which
invalidates the fundamental postulates of equilibrium statistical mechanics.
However, granular materials exhibit distributions of microscopic quantities
that are reproducible and often depend on only a few macroscopic parameters. We
explore the history of statistical ensemble ideas in the context of granular
materials, clarify the nature of such ensembles and their foundational
principles, highlight advances in testing key ideas, and discuss applications
of ensembles to analyze the collective behavior of granular materials
Local contact numbers in two dimensional packings of frictional disks
We analyze the local structure of two dimensional packings of frictional
disks numerically. We focus on the fractions x_i of particles that are in
contact with i neighbors, and systematically vary the confining pressure p and
friction coefficient \mu. We find that for all \mu, the fractions x_i exhibit
powerlaw scaling with p, which allows us to obtain an accurate estimate for x_i
at zero pressure. We uncover how these zero pressure fractions x_i vary with
\mu, and introduce a simple model that captures most of this variation. We also
probe the correlations between the contact numbers of neighboring particles.Comment: 4 pages, 5 figure
Structure and mechanics of active colloids
11 pages Acknowledgments MCM thanks Xingbo Yang and Lisa Manning for their contribution to some aspects of the work reviewed here and for fruitful discussions. MCM was supported by NSF-DMR-305184. MCM and AP acknowledge support by the NSF IGERT program through award NSF-DGE-1068780. MCM, AP and DY were additionally supported by the Soft Matter Program at Syracuse University. AP acknowledges use of the Syracuse University HTC Campus Grid which is supported by NSF award ACI-1341006. YF was supported by NSF grant DMR-1149266 and the Brandeis Center for Bioinspired Soft Materials, an NSF MRSEC, DMR-1420382.Peer reviewedPreprin
Dynamically generated patterns in dense suspensions of active filaments
We use Langevin dynamics simulations to study dynamical behaviour of a dense
planar layer of active semi-flexible filaments. Using the strength of active
force and the thermal persistence length as parameters, we map a detailed phase
diagram and identify several non-equilibrium phases in this system. In addition
to a slowly flowing melt phase, we observe that for sufficiently high activity,
collective flow accompanied by signatures of local polar and nematic order
appears in the system. This state is also characterised by strong density
fluctuations. Furthermore, we identify an activity-driven cross-over from this
state of coherently flowing bundles of filaments to a phase with no global
flow, formed by individual filaments coiled into rotating spirals. This
suggests a mechanism where the system responds to activity by changing the
shape of active agents, an effect with no analogue in systems of active
particles without internal degrees of freedom.Comment: extended and updated versio
Active swarms on a sphere
Here we show that coupling to curvature has profound effects on collective
motion in active systems, leading to patterns not observed in flat space.
Biological examples of such active motion in curved environments are numerous:
curvature and tissue folding are crucial during gastrulation, epithelial and
endothelial cells move on constantly growing, curved crypts and vili in the
gut, and the mammalian corneal epithelium grows in a steady-state vortex
pattern. On the physics side, droplets coated with actively driven microtubule
bundles show active nematic patterns. We study a model of self-propelled
particles with polar alignment on a sphere. Hallmarks of these motion patterns
are a polar vortex and a circulating band arising due to the incompatibility
between spherical topology and uniform motion - a consequence of the hairy ball
theorem. We present analytical results showing that frustration due to
curvature leads to stable elastic distortions storing energy in the band.Comment: 5 pages, 4 figures plus Supporting Informatio