Helical Swimming in Suspensions

To understand the effect of media heterogeneity in the swimming dynamics of bacteria, we study the motion of synthetic helical swimmers in suspensions. Low Reynolds number swimmers with helical tails are rotated by an external magnetic field acting on a magnet embedded within the swimmer. In addition to the swimming speed, both the drag force and the thrust force are measured directly using a force transducer. For the fluid, several suspensions of neutrally buoyant particles with varying volume concentrations were made and tested. Our experiments suggest that the swimming speed increases systematically with the concentration of particles in the fluid. Our experimental data are then compared with a theoretical model that accounts for the stress induced by moving spheres in the suspension around the helical tail and computes the changes in the values of the drag coefficients. The theoretical predictions are in good agreement with the experiments.