Low-Reynolds number swimming in gels

TL;DR

This paper investigates how microorganisms swim in gel-like environments using a two-fluid model, revealing that network dynamics, nonlinearities, and boundary conditions significantly influence swimming speeds and behaviors.

Contribution

It introduces a two-fluid continuum model for swimming in gels, highlighting the importance of nonlinearities, network deformation regimes, and boundary conditions on propulsion.

Findings

Swimming speeds can be enhanced in gels compared to Newtonian fluids.

Network deformation regimes depend on the coupling with solvent flows.

Boundary conditions critically affect swimming performance.

Abstract

Many microorganisms swim through gels, materials with nonzero zero-frequency elastic shear modulus, such as mucus. Biological gels are typically heterogeneous, containing both a structural scaffold (network) and a fluid solvent. We analyze the swimming of an infinite sheet undergoing transverse traveling wave deformations in the "two-fluid" model of a gel, which treats the network and solvent as two coupled elastic and viscous continuum phases. We show that geometric nonlinearities must be incorporated to obtain physically meaningful results. We identify a transition between regimes where the network deforms to follow solvent flows and where the network is stationary. Swimming speeds can be enhanced relative to Newtonian fluids when the network is stationary. Compressibility effects can also enhance swimming velocities. Finally, microscopic details of sheet-network interactions influence the boundary conditions between the sheet and network. The nature of these boundary conditions significantly impacts swimming speeds.