Viscotaxis : Microswimmer Navigation in Viscosity Gradients

Abstract

The survival of many microorganisms, like Leptospira or Spiroplasma bacteria, can depend on their ability to navigate towards regions of favorable viscosity. While this ability, called viscotaxis, has been observed in several bacterial experiments, the underlying mechanism remains unclear. We provide a framework to study viscotaxis of biological or synthetic self-propelled swimmers in slowly varying viscosity fields and show that suitable body shapes create viscotaxis based on a systematic asymmetry of viscous forces acting on a microswimmer. Our results shed new light on viscotaxis in Spiroplasma and Leptospira and suggest that dynamic body shape changes exhibited by both types of microorganisms may have an unrecognized functionality: to prevent them from drifting to low viscosity regions where they swim poorly. The present theory classifies microswimmers regarding their ability to show viscotaxis and can be used to design synthetic viscotactic swimmers, e.g., for delivering drugs to a target region distinguished by viscosity.

Theory of light-activated catalytic Janus particles

“…In analogy with biological systems, this directed spatial migra- * Electronic address: uspal@hawaii.edu tion can be considered a form of "taxis". Recent studies have investigated the rheotactic response of chemically active particles to hydrodynamic flow, [17][18][19][20] gravitactic response to the earth's gravitational field, [4,21,22] "viscotactic" response to viscosity gradients, [23] chemotactic response to gradients in the concentration of chemical "fuel", [24][25][26][27][28][29][30][31][32] and "thigmotactic" response to gradients in the material composition of bounding surfaces [33][34][35]. These various forms of taxis can be understood on the basis of the microscopic physics of how the ambient field couples to the activity and motion of the particle.…”

Roads to Smart Artificial Microswimmers

“…While some micro-organisms are known to move up (e.g., Leptospira [139] ) or down (e.g., E. coli [140] ) the viscosity gradients, this type of tactic behavior-viscotaxis-is less understood compared with other types of taxes discussed earlier. Liebchen et al [137] constructed model swimmers consisting of assemblies of spheres to demonstrate that viscotactic motion can emerge from an asymmetry of viscous forces on different parts of the swimmer (Figure 9c). These results hence suggest the association of dynamic body shape changes of micro-organisms with their viscotactic behaviors.…”

“…Reproduced with permission. [137] Copyright 2018, American Physical Society. d) Viscotactic response can also occur for axisymmetric squirmers (pushers, pullers, or neutral squirmers), which generally display negative viscotaxis.…”

Multi-species dynamical density functional theory for microswimmers: Derivation, orientational ordering, trapping potentials, and shear cells

“…(54) and (55) in combination with Eqs. (56) and (58), are proportional to ρ A (X, t) so that the above expressions do not diverge when the denominator vanishes.…”