Abstract

The locomotion of organisms in Newtonian fluids at low-Reynolds numbers displays very different features from that at large Reynolds numbers. In this regime, the viscous forces are dominant over the inertial ones and propulsion is possible only with non-time-reversible swimming strokes. In many situations of biological interest, however, small organisms are propelling themselves through non-Newtonian fluids such as mucus, biofilms or bio-polymer suspensions, all of which display highly viscoelastic properties. We investigated the effects of fluid viscoelasticity on the swimming velocity of a microorganism and on its dynamics in external flows. In our analysis, we employed the so called ``squirmer’’ model to study the motion of spherical ciliated organisms in a viscoelastic fluid. We first considered the motion of squirmers in quiescent viscoelastic fluids. Subsequently, we studied their dynamics in viscoelastic fluids with an imposed shear flow at infinity. Our results reveal that the coupling of self-propulsion and fluid viscoelasticity is responsible for a qualitatively different behaviour compared to that expected for a squirmer in a Newtonian fluid. Interestingly, it is found that squirmers suspended in sheared viscoelastic fluids attain a preferential swimming direction, which depends on their propulsion type (e.g. pusher, puller or neutral).