341 research outputs found
Controlling cell-matrix traction forces by extracellular geometry
We present a minimal continuum model of strongly adhering cells as active
contractile isotropic media and use the model to study the effect of the
geometry of the adhesion patch in controlling the spatial distribution of
traction and cellular stresses. Activity is introduced as a contractile, hence
negative, spatially homogeneous contribution to the pressure. The model shows
that patterning of adhesion regions can be used to control traction stress
distribution and yields several results consistent with experimental
observations. Specifically, the cell spread area is found to increase with
substrate stiffness and an analytic expression for the dependence is obtained
for circular cells. The correlation between the magnitude of traction stresses
and cell boundary curvature is also demonstrated and analyzed.Comment: 12 pages, 4 figure
Hydrodynamic and rheology of active polar filaments
The cytoskeleton provides eukaryotic cells with mechanical support and helps
them perform their biological functions. It is a network of semiflexible polar
protein filaments and many accessory proteins that bind to these filaments,
regulate their assembly, link them to organelles and continuously remodel the
network. Here we review recent theoretical work that aims to describe the
cytoskeleton as a polar continuum driven out of equilibrium by internal
chemical reactions. This work uses methods from soft condensed matter physics
and has led to the formulation of a general framework for the description of
the structure and rheology of active suspension of polar filaments and
molecular motors.Comment: 30 pages, 5 figures. To appear in "Cell Motility", Peter Lenz, ed.
(Springer, New York, 2007
Organization and instabilities of entangled active polar filaments
We study the dynamics of an entangled, isotropic solution of polar filaments
coupled by molecular motors which generate relative motion of the filaments in
two and three dimensions. We investigate the stability of the homogeneous state
for constant motor concentration taking into account excluded volume and
entanglement. At low filament density the system develops a density
instability, while at high filament density entanglement effects drive the
instability of orientational fluctuations.Comment: 4pages, 2 eps figure, revtex
Nonreciprocity as a generic route to traveling states
We examine a non-reciprocally coupled dynamical model of a mixture of two
diffusing species. We demonstrate that nonreciprocity, which is encoded in the
model via antagonistic cross diffusivities, provides a generic mechanism for
the emergence of traveling patterns in purely diffusive systems with
conservative dynamics. In the absence of non-reciprocity, the binary fluid
mixture undergoes a phase transition from a homogeneous mixed state to a
demixed state with spatially separated regions rich in one of the two
components. Above a critical value of the parameter tuning non-reciprocity, the
static demixed pattern acquires a finite velocity, resulting in a state that
breaks both spatial and time translational symmetry, as well as the reflection
parity of the static pattern. We elucidate the generic nature of the transition
to traveling patterns using a minimal model that can be studied analytically.
Our work has direct relevance to nonequilibrium assembly in mixtures of
chemically interacting colloids that are known to exhibit non-reciprocal
effective interactions, as well as to mixtures of active and passive agents
where traveling states of the type predicted here have been observed in
simulations. It also provides insight on transitions to traveling and
oscillatory states seen in a broad range of nonreciprocal systems with
non-conservative dynamics, from reaction-diffusion and prey-predators models to
multispecies mixtures of microorganisms with antagonistic interactions.Comment: 8 pages, 3 figure
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
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