31 research outputs found
Motility-induced phase separation and coarsening in active matter
Active systems, or active matter, are self-driven systems which live, or
function, far from equilibrium - a paradigmatic example which we focus on here
is provided by a suspension of self-motile particles. Active systems are far
from equilibrium because their microscopic constituents constantly consume
energy from the environment in order to do work, for instance to propel
themselves. The nonequilibrium nature of active matter leads to a variety of
non-trivial intriguing phenomena. An important one which has recently been the
subject of intense interest among biological and soft matter physicists is that
of the so-called "motility-induced phase separation", whereby self-propelled
particles accumulate into clusters in the absence of any explicit attractive
interactions between them. Here we review the physics of motility-induced phase
separation, and discuss this phenomenon within the framework of the classic
physics of phase separation and coarsening. We also discuss theories for
bacterial colonies where coarsening may be arrested. Most of this work will
focus on the case of run-and-tumble and active Brownian particles in the
absence of solvent-mediated hydrodynamic interactions - we will briefly discuss
at the end their role, which is not currently fully understood in this context.Comment: Contribution to the special issue "Coarsening dynamics", Comptes
Rendus de Physique, see
https://sites.google.com/site/ppoliti/crp-special-issu
A continuum theory of phase separation kinetics for active Brownian particles
Active Brownian particles (ABPs), when subject to purely repulsive
interactions, are known to undergo activity-induced phase separation broadly
resembling an equilibrium (attraction-induced) gas-liquid coexistence. Here we
present an accurate continuum theory for the dynamics of phase-separating ABPs,
derived by direct coarse-graining, capturing leading-order density gradient
terms alongside an effective bulk free energy. Such gradient terms do not obey
detailed balance; yet we find coarsening dynamics closely resembling that of
equilibrium phase separation. Our continuum theory is numerically compared to
large-scale direct simulations of ABPs and accurately accounts for domain
growth kinetics, domain topologies and coexistence densities
Lamellar ordering, droplet formation and phase inversion in exotic active emulsions
We study numerically the behaviour of a mixture of a passive isotropic fluid
and an active polar gel, in the presence of a surfactant favouring
emulsification. Focussing on parameters for which the underlying free energy
favours the lamellar phase in the passive limit, we show that the interplay
between nonequilibrium and thermodynamic forces creates a range of multifarious
exotic emulsions. When the active component is contractile (e.g., an actomyosin
solution), moderate activity enhances the efficiency of lamellar ordering,
whereas strong activity favours the creation of passive droplets within an
active matrix. For extensile activity (occurring, e.g., in microtubule-motor
suspensions), instead, we observe an emulsion of spontaneously rotating
droplets of different size. By tuning the overall composition, we can create
high internal phase emulsions, which undergo sudden phase inversion when
activity is switched off. Therefore, we find that activity provides a single
control parameter to design composite materials with a strikingly rich range of
morphologies.Comment: 15 pages: Manuscprit (4 figures) and SI (11 figures
Rheology of an inverted cholesteric droplet under shear flow
The dynamics of a quasi two-dimensional isotropic droplet in a cholesteric
liquid crystal medium under symmetric shear flow is studied by lattice
Boltzmann simulations. We consider a geometry in which the flow direction is
along the axis of the cholesteric, as this setup exhibits a significant
viscoelastic response to external stress. We find that the dynamics depends
upon the magnitude of the shear rate, the anchoring strength of the liquid
crystal at the droplet interface and the chirality. While for low shear rate
and weak interface anchoring the system shows a non-Newtonian behavior, a
Newtonian-like response is observed at high shear rate and strong interface
anchoring. This is investigated both by estimating the secondary flow profile,
namely a flow emerging along the out-of-plane direction (absent in fully
Newtonian fluids, such as water), and by monitoring defect formation and
dynamics which alter significantly the rheological response of the system.Comment: 14 pages, 5 figures. Contribution for the special issue Liquid
Crystal Rheolog