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

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