Experimental and theoretical studies of the fluid elasticity on the motion of macroscopic models of active helical swimmers

Abstract

This work presents experimental and theoretical studies on the locomotion of helical artificial swimmers at low Reynolds in both Newtonian and elastic ambient liquid. We examine the effect of fluid elasticity on the propulsive force and torque on the body and speed velocity of the swimmer in terms of two physical parameter: Deborah number and Strouhal number. For this end, some experiments with prototype microorganisms in creeping flow motion are conducted. In the experiments a macroscopic swimmer which propels itself by mimicking helical flagella are developed and tested. Three swimming models propelled by a helical tail with different wavelengths are investigated and their motions examined for both case: when ambient solvent is a pure Newtonian viscous fluid and when the base fluid is an elastic polymeric solution. In addition, we also apply the Slender Body Theory (SBT) and the method of regularized Stokeslet (RSM) in order to calculate theoretically the force and torque, as function of the Strouhal number, produced by the helical swimmer moving in a Newtonian fluid. The theoretical results are compared with experimental data and a very good agreement is observed specially for higher values of Strouhal within the error bars of the experimental data. In the case of a non-Newtonian ambient fluid, the flow problem of an Oldroyd-B elastic fluid is solved numerically using a computational code based on a finite element method (CFD).