718 research outputs found
Pattern formation of microtubules and motors: inelastic interaction of polar rods
We derive a model describing spatio-temporal organization of an array of
microtubules interacting via molecular motors. Starting from a stochastic model
of inelastic polar rods with a generic anisotropic interaction kernel we obtain
a set of equations for the local rods concentration and orientation. At large
enough mean density of rods and concentration of motors, the model describes
orientational instability. We demonstrate that the orientational instability
leads to the formation of vortices and (for large density and/or kernel
anisotropy) asters seen in recent experiments.Comment: 4 pages, 5 figures, to appear in Phys. Rev. E, Rapid Communication
Nematic and Polar order in Active Filament Solutions
Using a microscopic model of interacting polar biofilaments and motor
proteins, we characterize the phase diagram of both homogeneous and
inhomogeneous states in terms of experimental parameters. The polarity of motor
clusters is key in determining the organization of the filaments in homogeneous
isotropic, polarized and nematic states, while motor-induced bundling yields
spatially inhomogeneous structures.Comment: 4 pages. 3 figure
Bridging the microscopic and the hydrodynamic in active filament solutions
Hydrodynamic equations for an isotropic solution of active polar filaments
are derived from a microscopic mean-field model of the forces exchanged between
motors and filaments. We find that a spatial dependence of the motor stepping
rate along the filament is essential to drive bundle formation. A number of
differences arise as compared to hydrodynamics derived (earlier) from a
mesoscopic model where relative filament velocities were obtained on the basis
of symmetry considerations. Due to the anisotropy of filament diffusion, motors
are capable of generating net filament motion relative to the solvent. The
effect of this new term on the stability of the homogeneous state is
investigated.Comment: 7 pages, 2 figures, submitted to Europhys. Let
Rheology of Active Filament Solutions
We study the viscoelasticity of an active solution of polar biofilaments and
motor proteins. Using a molecular model, we derive the constitutive equations
for the stress tensor in the isotropic phase and in phases with liquid
crystalline order. The stress relaxation in the various phases is discussed.
Contractile activity is responsible for a spectacular difference in the
viscoelastic properties on opposite sides of the order-disorder transition.Comment: 4 pages, 1 figur
Spontaneous flow transition in active polar gels
We study theoretically the effects of confinement on active polar gels such
as the actin network of eukaryotic cells. Using generalized hydrodynamics
equations derived for active gels, we predict, in the case of quasi
one-dimensional geometry, a spontaneous flow transition from a homogeneously
polarized immobile state for small thicknesses, to a perturbed flowing state
for larger thicknesses. The transition is not driven by an external field but
by the activity of the system. We suggest several possible experimental
realizations.Comment: 7 pages, 3 figures. To appear in Europhys. Let
Generic phase diagram of active polar films
We study theoretically the phase diagram of compressible active polar gels
such as the actin network of eukaryotic cells. Using generalized hydrodynamics
equations, we perform a linear stability analysis of the uniform states in the
case of an infinite bidimensional active gel to obtain the dynamic phase
diagram of active polar films. We predict in particular modulated flowing
phases, and a macroscopic phase separation at high activity. This qualitatively
accounts for experimental observations of various active systems, such as
acto-myosin gels, microtubules and kinesins in vitro solutions, or swimming
bacterial colonies.Comment: 4 pages, 1 figur
Actively Contracting Bundles of Polar Filaments
We introduce a phenomenological model to study the properties of bundles of
polar filaments which interact via active elements. The stability of the
homogeneous state, the attractors of the dynamics in the unstable regime and
the tensile stress generated in the bundle are discussed. We find that the
interaction of parallel filaments can induce unstable behavior and is
responsible for active contraction and tension in the bundle. Interaction
between antiparallel filaments leads to filament sorting. Our model could apply
to simple contractile structures in cells such as stress fibers.Comment: 4 pages, 4 figures, RevTex, to appear in Phys. Rev. Let
- …