43 research outputs found
Statistical hydrodynamics of ordered suspensions of self-propelled particles: waves, giant number fluctuations and instabilities
General principles of symmetry and conservation are used to construct the hydrodynamic equations for orientationally ordered suspensions of self-propelled particles (SPPs). Without knowledge of the microscopic origins of the ordering or the mechanisms of self-propulsion, we are able to make a number of striking, testable predictions for the properties of these nonequilibrium phases of matter. These include: novel wavelike excitations in vectorially ordered suspensions; the absolute instability of nematic SPP suspensions at long wavelengths; the convective instability of low-Reynolds-number vector-ordered suspensions; and giant number fluctuations in vector-ordered SPP suspensions
Classical XY model with conserved angular momentum is an archetypal non-Newtonian fluid
We find that the classical one-dimensional (1D) XY model, with angular-momentum-conserving Langevin dynamics, mimics the non-Newtonian flow regimes characteristic of soft matter when subjected to counter-rotating boundaries. An elaborate steady-state phase diagram has continuous and first-order transitions between states of uniform flow, shear-banding, solid-fluid coexistence and slip-planes. Results of numerically studies and a concise mean-field constitutive relation, offer a paradigm for diverse non-equilibrium complex fluids
Active nematics on a substrate: giant number fluctuations and long-time tails
We construct the equations of motion for the coupled dynamics of order
parameter and concentration for the nematic phase of driven particles on a
solid surface, and show that they imply (i) giant number fluctuations, with a
standard deviation proportional to the mean and (ii) long-time tails in the autocorrelation of the particle velocities in dimensions
despite the absence of a hydrodynamic velocity field. Our predictions can be
tested in experiments on aggregates of amoeboid cells as well as on layers of
agitated granular matter.Comment: Submitted to Europhys Lett 26 Aug 200
Hydrodynamic fluctuations and instabilities in ordered suspensions of self-propelled particles
We construct the hydrodynamic equations for {\em suspensions} of
self-propelled particles (SPPs) with spontaneous orientational order, and make
a number of striking, testable predictions:(i) SPP suspensions with the
symmetry of a true {\em nematic} are {\em always} absolutely unstable at long
wavelengths.(ii) SPP suspensions with {\em polar}, i.e., head-tail {\em
asymmetric}, order support novel propagating modes at long wavelengths,
coupling orientation, flow, and concentration. (iii) In a wavenumber regime
accessible only in low Reynolds number systems such as bacteria, polar-ordered
suspensions are invariably convectively unstable.(iv) The variance in the
number N of particles, divided by the mean , diverges as in
polar-ordered SPP suspensions.Comment: submitted to Phys Rev Let
A Dynamic Renormalization Group Study of Active Nematics
We carry out a systematic construction of the coarse-grained dynamical
equation of motion for the orientational order parameter for a two-dimensional
active nematic, that is a nonequilibrium steady state with uniaxial, apolar
orientational order. Using the dynamical renormalization group, we show that
the leading nonlinearities in this equation are marginally \textit{irrelevant}.
We discover a special limit of parameters in which the equation of motion for
the angle field of bears a close relation to the 2d stochastic Burgers
equation. We find nevertheless that, unlike for the Burgers problem, the
nonlinearity is marginally irrelevant even in this special limit, as a result
of of a hidden fluctuation-dissipation relation. 2d active nematics therefore
have quasi-long-range order, just like their equilibrium counterpartsComment: 31 pages 6 figure
Rheology of Active-Particle Suspensions
We study the interplay of activity, order and flow through a set of
coarse-grained equations governing the hydrodynamic velocity, concentration and
stress fields in a suspension of active, energy-dissipating particles. We make
several predictions for the rheology of such systems, which can be tested on
bacterial suspensions, cell extracts with motors and filaments, or artificial
machines in a fluid. The phenomena of cytoplasmic streaming, elastotaxis and
active mechanosensing find natural explanations within our model.Comment: 3 eps figures, submitted to Phys Rev Let
Travelling waves in a drifting flux lattice
Starting from the time-dependent Ginzburg-Landau (TDGL) equations for a type
II superconductor, we derive the equations of motion for the displacement field
of a moving vortex lattice without inertia or pinning. We show that it is
linearly stable and, surprisingly, that it supports wavelike long-wavelength
excitations arising not from inertia or elasticity but from the
strain-dependent mobility of the moving lattice. It should be possible to image
these waves, whose speeds are a few \mu m/s, using fast scanning tunnelling
microscopy.Comment: 4 pages, revtex, 2 .eps figures imbedded in paper, title shortened,
minor textual change
A bacterial ratchet motor
Self-propelling bacteria are a dream of nano-technology. These unicellular
organisms are not just capable of living and reproducing, but they can swim
very efficiently, sense the environment and look for food, all packaged in a
body measuring a few microns. Before such perfect machines could be
artificially assembled, researchers are beginning to explore new ways to
harness bacteria as propelling units for micro-devices. Proposed strategies
require the careful task of aligning and binding bacterial cells on synthetic
surfaces in order to have them work cooperatively. Here we show that asymmetric
micro-gears can spontaneously rotate when immersed in an active bacterial bath.
The propulsion mechanism is provided by the self assembly of motile Escherichia
coli cells along the saw-toothed boundaries of a nano-fabricated rotor. Our
results highlight the technological implications of active matter's ability to
overcome the restrictions imposed by the second law of thermodynamics on
equilibrium passive fluids.Comment: 4 pages, 3 figure