Enhanced self-propulsion of a sphere-dimer in viscoelastic fluid.

Micro-swimmers often have to encounter a medium that exhibits non-Newtonian behaviour. To understand the effect of complex environments on the propulsion dynamics of swimmers, here we have investigated a self-propelled sphere-dimer in a viscoelastic medium, using a coarse-grained hybrid mesoscopic simulation technique. We have shown that a viscoelastic fluid can result in the enhancement of swimming speed, as compared to the speed in a Newtonian fluid with the same viscosity. A non-linear response in the dimer velocity is seen for higher Péclet numbers in viscoelastic fluids. With help of various dynamical quantities, we have shown that the observed non-linear response of the directed velocity is associated with the micro-structural properties of the fluid. These include the alignment of the fluid elements and the density inhomogeneity around the moving dimer. The enhancement of self-propulsion velocity has been probed in detail, and the factors affecting the propulsion are identified.