Dynamics of microswimmers near a liquid–liquid interface with viscosity difference

Abstract

Transport of material across liquid interfaces is ubiquitous for living cells and is also a crucial step in drug delivery and in many industrial processes. The fluids that are present on either side of the interfaces will usually have different viscosities. We present a physical model for the dynamics of microswimmers near a soft and penetrable interface that we solve using computer simulations of Navier–Stokes flows. The literature contains studies of similar isoviscous fluid systems, where the two fluids have the same viscosity. Here, we extend this to the more general case where they have different viscosities. In particular, we investigate the dynamics of swimmers approaching a fluid–fluid interface between phase-separated fluids with distinct viscosities. We find that the incoming angle, viscosity ratio, and swimming type (i.e., pusher, puller, or neutral) strongly influence the collision, resulting in four distinct dynamical modes: bouncing, sliding, penetrating, and hovering. The former three modes are also observed for isoviscous systems, while the hovering, in which strong pullers swim parallel to the interface at a non-zero distance, requires mismatched viscosities. Furthermore, swimmers exhibit a preference for lower viscosity fluids, known as viscotaxis. This implies that, for a wide distribution of contact angles, more swimmers will transition into the low-viscosity environment than vice versa. Consequently, a swimmer starting in a low-viscosity fluid is more likely to bounce back at the interface, while a swimmer in a high-viscosity fluid is more likely to penetrate the interface and enter the lower viscosity fluid.