Swimming of microorganisms in complex fluids; Part 2: Interactions

Bacteria exhibit various collective motions, many of which have important implications for the properties of the suspension they dwell in. The flows created influence their mutual interactions and modify the rheology of their suspensions. Although the flow field around real bacteria has been studied, a few have studied the effect of changing geometrical parameters and fluid rheology. We carry out experiments using the techniques described in Part 1 to study the interactions of swimmers in both purely viscous and viscoelastic fluids for a range of helical wavelength, λ, Deborah number, De, and two head geometries. We found strikingly different behaviors when the fluid has elastic properties; for instance, instead of attraction, we observe repulsion. Also, the orbiting behavior observed in a Newtonian fluid is no longer seen in the Boger fluid. We rationalize the changes in the interactions by analyzing the velocity fields around swimmers. The dominant Stokeslet-like velocity field observed in Newtonian fluids is significantly modified by elastic effects. Many types of interactions will be shown and discussed, addressing the effects of elasticity. The work provides insights into understanding predator-prey interactions and complex bio-fluid systems of flagellar microorganisms.