Abstract
Particle–wall interactions have broad biological and technological applications. In particular, some artificial microswimmers capitalize on their translation–rotation coupling near a wall to generate directed propulsion. Emerging biomedical applications of these microswimmers in complex biological fluids prompt questions on the impact of non-Newtonian rheology on their propulsion. In this work, we report some intriguing effects of shear-thinning rheology, a ubiquitous non-Newtonian behaviour of biological fluids, on the translation–rotation coupling of a particle near a wall. One particularly interesting feature revealed here is that the wall-induced translation by rotation can occur in a direction opposite to what might be intuitively expected for an object rolling on a solid substrate. We elucidate the underlying physical mechanism and discuss its implications on the design of micromachines and bacterial motion near walls in complex fluids.
Highlights
The motion of microparticles near boundaries is of broad interest because the proximity of boundaries is all but unavoidable in most real situations
We report some intriguing effects of shear-thinning rheology on the translation–rotation coupling of a particle near a wall
Our results suggest a plausible mechanism for the observed directional change in circular motion of swimming bacteria near a solid wall in complex fluids
Summary
The motion of microparticles near boundaries is of broad interest because the proximity of boundaries is all but unavoidable in most real situations. A class of artificial microswimmers, known as surface walkers or microrollers, exploit their interactions with nearby surfaces to generate directed propulsion (Tierno et al 2008; Sing et al 2010; Driscoll et al 2017). These microswimmers are driven into rotation typically by external magnetic fields. We report some intriguing effects of shear-thinning rheology on the translation–rotation coupling of a particle near a wall Such coupling is relevant to the propulsion of microrollers and the near-wall dynamics of swimming bacteria in complex fluids. The features reported may inspire novel techniques for particle manipulation in microfluidics with non-Newtonian flows and microrheological measurements
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