Boundaries control active channel flows

TL;DR

This paper investigates how boundary conditions, such as surface anchoring and substrate drag, can be used to control spontaneous active fluid flows in channels, revealing the role of symmetry and activity levels in flow states.

Contribution

It introduces a numerical and analytical study showing boundary patterning can control active fluid flow regimes and transitions in a 2D channel.

Findings

Flow states depend on activity and boundary anchoring.

Coherent flow occurs only with weak anchoring and moderate activity.

Boundary patterning can manipulate active flow properties.

Abstract

Boundary conditions dictate how fluids, including liquid crystals, flow when pumped through a channel. Can boundary conditions also be used to control internally driven active fluids that generate flows spontaneously? By using numerical simulations and stability analysis we explore how surface anchoring of active agents at the boundaries and substrate drag can be used to rectify coherent flow of an active polar fluid in a 2D channel. Upon increasing activity, a succession of dynamical states is obtained, from laminar flow to vortex arrays to eventual turbulence, that are controlled by the interplay between the hydrodynamic screening length and the extrapolation length quantifying the anchoring strength of the orientational order parameter. We highlight the key role of symmetry in both flow and order and show that coherent laminar flow with net throughput is only possible for weak anchoring and intermediate activity. Our work demonstrates the possibility of controlling the nature and properties of active flows in a channel simply by patterning the confining boundaries.