Adriano Tiribocchi — Università di Padova#
Phase separation in polar active liquid crystals.#
Active fluids are inherently far-from-equilibrium systems whose internal self-propelled constituents convert chemical energy into work [1]. This non-equilibrium feature unveils several novel striking effects, such as giant density fluctuations [2], spontaneous flows [3] and unexpected phase separation [4,5]. Of particular importance is the so-called motility induced phase separation (MIPS), a phenomenon which is attracting a remarkable amount of interest in the physics community. If the propulsion speed of the motile particles decreases fast enough with the density due to the crowding, phase separation occurs even without any attractive interaction [6]. Several theoretical models, both particle [5,7] and continuum-based [5,6,8], have been proposed in order to shed light on the physics of MIPS. In particular simulations show that, when hydrodynamic interactions are negligible, the kinetics of domain growth exhibits a diffusive behavior [5].
On the other hand, motile particles with orientational order (such as bacteria, whose shape can be assumed as rod-like with no head-tail symmetry) phase separate too. Their intrinsic tendency to align each other [9], in presence of fluctuations in the local density, can induce phase separation [2]. By using a relatively simple model (already adopted to describe active polar liquid crystals [10]) in which the degree of order (namely the alignment among particles) is encoded in a vector field whose evolution is governed by advection-relaxation-like equations, the concentration of active material is described by using a scalar field governed by a Cahn-Hilliard-like equation and hydrodynamic interactions are described by the Navier-Stokes equation, we will elucidate how hydrodynamics affect domain coarsening and will show that, regardless the mechanisms responsible for generating fluid flows, phase separation is arrested. In particular we will show the physics of active phase separation for mixtures of active (namely contractile and extensile) particles at symmetric and off-symmetric concentration and for mixtures of active and passive particles. 
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