Swimming of microorganisms in viscoelastic fluids
Many natural habitats of biological microswimmers include complex fluids whose mechanical response is strongly non-Newtonian. Recent attempts to understand swimming in such fluids produced a series of seemingly contradictory results. Especially, it is currently debated whether swimming in dilute polymer solutions would be faster or slower than in Newtonian fluids like water.
One of the classical models to study swimming of microorganisms is a 2D infinite periodic waving sheet model introduced by G. I. Taylor. For small-amplitude swimming it was shown analytically that viscoelasticity of the suspending fluid reduces the propulsion speed, while simulations of a finite-size version of the same model predicted an increase of the propulsion speed followed by a decrease as the fluid becomes progressively more elastic.
I will present a mechanism that explains the reduction of the propulsion speed for the waving-sheet model. I will show the results of analytical and exact numerical calculations of large-amplitude swimming of sheets with various waveforms and confirm our mechanism. These results provide a framework to understand the mechanism of swimming in viscoelastic media.
In the second part of the talk I will discuss how this theory relates to real bacteria swimming in polymer solutions.