Theory and Lattice Boltzmann simulation of active fluids
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Date
26/11/2019Author
De Magistris, Giulio
Metadata
Abstract
Active fluids are far from equilibrium systems, nominally highly dense suspensions
of elongated elements that confer to the liquid orientational order, similar in
nature to the one found in liquid crystals, and an internal stirring that they exert
through the consumption of some fuel reservoir { typically AdenosineTriPhosphate(ATP).
The interplay between the orientational order and the active behaviour gives
rise to non-trivial dynamics, order-disorder out-of-equilibrium transitions, and
dynamical transitions to non-equilibrium steady states.
In this work we will discuss active fluids in the framework of the hydrodynamic
theory of active nematics and active polar fluids, where the structural order is
characterised by an elastic formalism drawn from the theory for liquid crystals,
and the activity is obtained from the coarse graining of the microscopics. Our aim
was to analyse and characterize large scale behaviours in multi-phase systems.
We studied the dynamics of water phases embedded in active polar fluids,
or rather 'inverse droplets', and considered how these move by effect of the
surrounding active medium. We found two types of motile transitions, depending
on the specifics of the boundary conditions of the orientation profile at the
interface between the passive and the active phase.
We also considered quenches of disordered mixtures of active nematic and passive
isotropic fluids. In these systems we observed cases of arrested phase separation,
with the coarsening saturating at finite length scales depending on the strength
of the activity. We characterized this behaviour in terms of an active capillary
number, resulting from the relative strength of active interface effects and surface
tension.