2,749 research outputs found
Non-equilibrium systems have steady-state distributions and non-steady dynamics
We search for steady states in a class of fluctuating and driven physical
systems that exhibit sustained currents. We find that the physical concept of a
steady state, well known for systems at equilibrium, must be generalised to
describe such systems. In these, the generalisation of a steady state is
associated with a stationary probability density of micro-states and a
deterministic dynamical system whose trajectories the system follows on
average. These trajectories are a manifestation of non-stationary macroscopic
currents observed in these systems. We determine precise conditions for the
steady state to exist as well as the requirements for it to be stable. We
illustrate this with some examples.Comment: 12+ pages, 2 figure
Sheared active fluids: thickening, thinning and vanishing viscosity
We analyze the behavior of a suspension of active polar particles under
shear. In the absence of external forces, orientationally ordered active
particles are known to exhibit a transition to a state of non-uniform
polarization and spontaneous flow. Such a transition results from the interplay
between elastic stresses, due to the liquid crystallinity of the suspension,
and internal active stresses. In the presence of an external shear we find an
extremely rich variety of phenomena, including an effective reduction
(increase) in the apparent viscosity depending on the nature of the active
stresses and the flow-alignment property of the particles, as well as more
exotic behaviors such as a non-monotonic stress/strain-rate relation and yield
stress for large activities.Comment: 10 pages, 10 figure
The dynamics of a self-phoretic Janus swimmer near a wall
We study the effect of a nearby planar wall on the propulsion of a phoretic
Janus micro-swimmer driven by asymmetric reactions on its surface which absorb
reactants and generate products. We show that the behaviour of these swimmers
near a wall can be classified the swimmers are
absorbing or producing reaction solutes
their swimming directions are such that the inert or active face is at the
front. We find that the wall-induced solute gradients always promote swimmer
propulsion along the wall while the effect of hydrodynamics leads to
re-orientation of the swimming direction away from the wall.Comment: 6 pages, 6 figure
Synchronisation and liquid crystalline order in soft active fluids
We introduce a phenomenological theory for a new class of soft active fluids,
with the ability to synchronise. Our theoretical framework describes the
macroscopic behaviour of a collection of interacting anisotropic elements with
cyclic internal dynamics and a periodic phase variable. This system (i) can
spontaneously undergo a transition to a state with macroscopic orientational
order, with the elements aligned: a liquid crystal, (ii) attain another broken
symmetry state characterised by synchronisation of their phase variables or
(iii) a combination of both types of order. We derive the equations describing
a spatially homogeneous system and also study the hydrodynamic fluctuations of
the soft modes in some of the ordered states. We find that synchronisation can
promote the transition to a state with orientational order; and vice-versa.
Finally, we provide an explicit microscopic realisation: a suspension of
micro-swimmers driven by cyclic strokes.Comment: 5 pages, 3 figure
Hydrodynamic interactions in dense active suspensions: from polar order to dynamical clusters
We study the role of hydrodynamic interactions in the collective behaviour of
collections of microscopic active particles suspended in a fluid. We introduce
a novel calculational framework that allows us to separate the different
contributions to their collective dynamics from hydrodynamic interactions on
different length scales. Hence we are able to systematically show that
lubrication forces when the particles are very close to each other play as
important a role as long-range hydrodynamic interactions in determining their
many-body behaviour. We find that motility-induced phase separation is
suppressed by near-field interactions, leading to open gel-like clusters rather
than dense clusters. Interestingly, we find a globally polar ordered phase
appears for neutral swimmers with no force dipole that is enhanced by near
field lubrication forces in which the collision process rather than long-range
interaction dominates the alignment mechanism.Comment: 7 pages, 4 figure
Food Insecurity, Family Structure and Agricultural Productivity: the role of Social Capital in Nigeria.
Replaced with revised version of paper 07/21/10.Food Security and Poverty,
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
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
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