Shear flow induced isotropic to nematic transition in a suspension of active filaments

– We study the effects of externally applied shear flow on a model of suspensions of motors and filaments, via the equations of active hydrodynamics [PRL 89 (2002) 058101; 92 (2004) 118101]. In the absence of shear, the orientationally ordered phase of both polar and apolar active particles is always unstable at zero-wavenumber. An imposed steady shear large enough to overcome the active stresses stabilises both apolar and moving polar phases. Our work is relevant to in vitro studies of active filaments, the reorientation of endothelial cells subject to shear flow and shear-induced motility of attached cells. The collective behaviour of suspensions of active particles() is fundamentally different from that of their passive counterparts, as highlighted by recent studies on hydrodynamic instabilities [1,2], rheology [3] and steady state patterns [4–6] of active particle systems. Recent experimental studies on model systems such as motor-filament extracts [7,8] and suspensions of swimming bacteria [9–11] or spermatocytes [12] have also reported strong departures from equilibrium behaviour. One surprising prediction [1] is that, unlike in passive suspensions, long-range uniaxial orientational order in active Stokesian() suspensions of polar particles is always destroyed by a hydrodynamic instability. This is because the flow generated by a small, long-wavelength splay or bend deformation imposed on an oriented configuration always acts instantaneously (in the Stokesian regime) to further increase the deformation [1, 13]. () E-mail: [email protected] () E-mail: [email protected] () E-mail: [email protected] ()By active particle we mean an object which absorbs energy from its surroundings and dissipates it in the process of carrying out internal movements; examples include self-propelled organisms, motor-filament complexes, and agitated monolayers of granular particles. ()where viscosity dominates and inertia is ignored