Sheared active fluids: Thickening, thinning, and vanishing viscosity

Abstract

We analyze the behavior of a suspension of active polar particles under shear. In the absence of external forces, orientationally ordered active particles are known to exhibit a transition to a state of non-uniform polarization and spontaneous flow. Such a transition results from the interplay between elastic stresses, due to the liquid crystallinity of the suspension, and internal active stresses. In the presence of an external shear we find an extremely rich variety of phenomena, including an effective reduction (increase) in the apparent viscosity depending on the nature of the active stresses and the flow-alignment property of the particles, as well as more exotic behaviors such as a non-monotonic stress/strain-rate relation and yield stress for large activities.

Turning Bacteria Suspensions into Superfluids

“…Remarkably, at low shear rate, these theories predict a Newtonian plateau with a viscosity decreasing linearly with concentration [19][20][21]. On the other hand, phenomenological theories were also proposed to describe macroscopically active suspensions via a coupling of hydrodynamic equations with polar and/or nematic order parameters [2, 5, 6,[22][23][24]. A striking outcome of these theories is that for a set of coupling parameters rendering essentially a high swimming activity, a self-organized motive macroscopic flow may show up in response to shear [22][23][24].…”

“…On the other hand, phenomenological theories were also proposed to describe macroscopically active suspensions via a coupling of hydrodynamic equations with polar and/or nematic order parameters [2, 5, 6,[22][23][24]. A striking outcome of these theories is that for a set of coupling parameters rendering essentially a high swimming activity, a self-organized motive macroscopic flow may show up in response to shear [22][23][24]. This onset of a dissipationless current is described in analogy with the super-fluidity transition [22,23] of liquids.…”

Geometry-dependent viscosity reduction in sheared active fluids

“…In particular, recent shear experiments suggest that Escherichia coli bacteria can create effectively inviscid flow if their concentration and activity are sufficiently large to support coherent collective swimming [18]. From a theory perspective, it is desirable to formulate a minimal hydrodynamic model that is analytically tractable and can account for all the aforementioned experimental observations without overfitting.Previous theoretical work [19][20][21][22][23][24] identified potential viscosity reduction mechanisms [15,18] in certain classes of active suspensions, but the complexity and specific nature of the underlying multi-field models have made analytical insight, time-resolved dynamical studies and comparison with experiment challenging. To better understand the general conditions under which active fluids can develop spontaneous symmetry-breaking and quasi-inviscid behavior, we pursue here an alternative approach by focusing on the generic phenomenological properties of non-Newtonian fluids that exhibit biologically, chemically or physically driven pattern formation.…”

Nonlinear spontaneous symmetry breaking in active polar films

“…Thus, active liquid crystal films, beyond a critical film thickness or activity can generate motion even in the absence of externally applied forces [17] , in contrast to their passive counterparts [15]. The transition from stationary to flowing states and resulting non-equilibrium phase diagrams are usually obtained via linear stability analyses [16,17,[19][20][21][22]. Active polar fluids have a rich spectrum of behaviours [19][20][21][22].…”

“…The transition from stationary to flowing states and resulting non-equilibrium phase diagrams are usually obtained via linear stability analyses [16,17,[19][20][21][22]. Active polar fluids have a rich spectrum of behaviours [19][20][21][22]. Indeed, like active nematics [17], they exhibit steady spontaneous flow.…”