Dynamics of self-propelled particles in viscoelastic fluids

The motion of many natural microswimmers, e.g. bacteria and spermatozoa, commonly takes place in viscoelastic fluids and under confinement close to solid walls.

The understanding of their swimming mechanisms has triggered a lot of experimental and theoretical work in recent years as well as the development of self-propelled colloidal particles. Although the motion of such synthetic microswimmers in Newtonian fluids has been extensively studied [1], so far only few investigations have focused on the swimming of microorganisms in viscoelastic fluids [2].

Here, I will present some recent experiments on the dynamics of spherical colloidal microswimmers in viscoelastic fluids, which are self-propelled by local demixing of a critical binary polymer mixture induced by laser illumination. Unlike the motion in Newtonian liquids, we observe a pronounced enhancement of rotational diffusion with increasing particle velocity, even in the regime where the drag force on the particle exerted by the fluid obeys the Stokes law [3].

We demonstrate that this effect originates from the coupling between the directed motion of the particle, and the slow microstructural relaxation of the surrounding fluid [4]. Furthermore, we show that such a coupling has dramatic consequences for the particle translational and rotational dynamics close to solid walls, as well as for collective motion in crowded environments.

[1] C. Bechinger, R. Di Leonardo, H. Löwen, C. Reichhardt, G. Volpe, and G. Volpe, Rev. Mod. Phys. 88, 045006 (2016).

[2] A. Patteson, A. Gopinath, and P. E. Arratia, Curr. Opin. Colloid Interface Sci., 21,86 (2016).

[3] J. R. Gomez-Solano, A. Blokhuis, and C. Bechinger, Phys. Rev. Lett. 

116, 138301 (2016).

[4] J. R. Gomez-Solano, and C. Bechinger, New J. Physics 17, 103032 (2015).