Geometry-dependent viscosity reduction in sheared active fluids

TL;DR

This paper models active fluids to understand how flow patterns and viscosity reduction depend on geometry, revealing conditions for spontaneous symmetry breaking and negative viscosity states that could be used for energy harvesting.

Contribution

It introduces an analytical and computational framework for studying flow in active fluids, highlighting the role of geometry in viscosity reduction and vortex dynamics.

Findings

Identification of conditions for spontaneous flow symmetry breaking

Demonstration of geometry-dependent viscosity reduction

Prediction of negative-viscosity states in confined suspensions

Abstract

We investigate flow pattern formation and viscosity reduction mechanisms in active fluids by studying a generalized Navier-Stokes model that captures the experimentally observed bulk vortex dynamics in microbial suspensions. We present exact analytical solutions including stress-free vortex lattices and introduce a computational framework that allows the efficient treatment of previously intractable higher-order shear boundary conditions. Large-scale parameter scans identify the conditions for spontaneous flow symmetry breaking, geometry-dependent viscosity reduction and negative-viscosity states amenable to energy harvesting in confined suspensions. The theory uses only generic assumptions about the symmetries and long-wavelength structure of active stress tensors, suggesting that inviscid phases may be achievable in a broad class of non-equilibrium fluids by tuning confinement geometry and pattern scale selection.