Rheology of active polar emulsions: from linear to unidirectional and inviscid flow, and intermittent viscosity

Abstract

The rheological behaviour of an emulsion made of an active polar component and an isotropic passive fluid is studied by lattice Boltzmann methods. Different flow regimes are found by varying the values of shear rate and extensile activity (occurring, e.g., in microtubule-motor suspensions). By increasing activity, a first transition occurs from linear flow regime to spontaneous persistent unidirectional macro-scale flow, followed by another transition either to (low shear) intermittent flow regime with coexistence of states with positive, negative, and vanishing apparent viscosity, or to (high shear) symmetric shear thinning regime. The different behaviours can be explained in terms of the dynamics of the polarization field close to the walls. A maximum entropy production principle selects the most likely states in the intermittent regime. 0

Soft channel formation and symmetry breaking in exotic active emulsions

“…Finally, we found that the effective viscosity of the system increases together with the intensity of the contractile activity. Such dynamic behavior is drastically different from that of the extensile mixture under shear, which is found to exhibit a multi-stable dynamics characterized by states at null and negative viscosity 36 , in agreement with the experimental results of Lopez and Guo.…”

“…The equilibrium properties of the system are encoded in a modified Landau–Brazovskii 34 – 36 , 39 , 40 free-energy functional that can be expressed as the first contribution defines the thermodynamic property of the binary mixture: The first term allows for the segregation of the two phases when , as the free energy has two minima at and , respectively corresponding to the passive and active phase, while . The gradient terms determine the surface tension.…”

“…In particular, when , the polarization points towards the decreasing values of . The ground state defined by the free energy functional is characterized by a transition from the isotropic towards the lamellar phase for 36 , with the polarization confined in only one of the two phase and normally anchored to the interfaces. We note that the presence of a strong homeotropic interface anchoring is crucial to ensure the stability of the lamellar phase in active mixtures.…”

“…Simulations based on active gel theory have been proved to be able to qualitatively replicate the dynamics of active droplets—from the regular motion of topological defects 8 , 29 , to the turbulent regime 15 , 30 —and predict new motility modes 29 , 31 – 33 . In this paper we consider a theoretical model of an active binary mixture 34 – 36 , to numerically study the rheological response of contractile mixture under an externally imposed shear.…”

Pattern formation in active model C with anchoring: bands, aster networks, and foams

“…A suspension of cytoskeletal filaments and motor proteins is a paradigmatic example of active matter. [1][2][3][4] This system is active as the filaments can be moved with respect to each other by molecular motors, which consume chemical energy in the form of ATP and drive the system out of equilibrium. A typical example of such motors is given by kinesin 5 and its synthetic analogues that were shown to cross-link microtubules (MTs) and to push them apart.…”