Swimming of microorganisms in complex fluids; Part 1: Viscoelastic effects

Most motile microorganisms dwell in complex fluids. We aim to understand the dynamics of microbial swimming using a macroscopic force-free, torque-free robot that emulates the swimming strategy of real bacteria. In particular, to address the effects of viscoelasticity, we use Boger fluids (constant viscosity but elastic). Additionally, we quantified the flow field around the swimmers using standard Particle image velocimetry techniques (PIV). The measurements are conducted for a range of helical wavelength, λ, Deborah number, De, and two head geometries. We observe a significant swimming speed enhancement over the varied range of parameters. To understand this behavior, we decompose the thrust and drag forces into viscous and elastic contributions. Using resistive force theory, we can predict the enhancement and impedance in swimming speeds. The appearance of negative wakes in elastic fluids can be associated with improved swimmer performance. This provides insights into propulsion mechanisms of micro-swimmers in complex fluid environments and the kinematics of swimming gaits, extending it to hydrodynamic interactions with their surroundings and other swimmers (these issues are discussed in Part 2 of this presentation).