Auto-electrophoresis in non-Newtonian media: interaction of rheology and electrocatalytic parameters

Recent findings of possible applications of bio-friendly synthetic self-phoretic swimmers, have motivated the researchers in investigating the various motion-generating mechanisms to optimize the operating characteristics of the same. In this paper we model the auto-electrophoretic motion of a bimetallic (Au-Pt) spherical swimmer in a non-Newtonian medium. In view of the fact that many bio-fluids closely follow the generalized Newtonian rheology, the rheology of the surrounding swimming medium is considered to follow the Carreau-Yasuda viscosity model. Further to capture the experimentally observed effects of peroxide concentration and feedback of generated proton concentration on the surface cation flux, we incorporate a Michaelis-Menten like surface reaction kinetics. The electrocatalytic efficiency defined as a ratio between the cost of swimming of an equivalent passively dragged particle and the input chemical energy via surface reaction, is utilized to assess the influence of the rheological parameters on the swimmer performance. The results indicate that the shear thinning and shear thickening nature of the surrounding fluid causes an enhancement and reduction in swimmer velocity, respectively. However, an increase in the shear thinning effect does not always guarantee an efficient operation of the microswimmer. A competition between the particle velocity and reduction in the drag force on the particle, decides whether the propulsion efficiency will have a relative augmentation or attenuation with varying power law index. We further report the existence of maximum efficiency points for some specific range of Weissenberg number and optimum power law indices. Moreover the behaviour of such optimum operating conditions strongly depends on the non-trivial and highly coupled interplay among the rheology and electrocatalytic parameters.