Upstream Rheotaxis of Catalytic Janus Spheres

Fluid flow is ubiquitous in many environments that form habitats for microorganisms. Therefore, it is not surprising that both biological and artificial microswimmers show responses to flows that are determined by the interplay of chemical and physical factors. In particular, to deepen the understanding of how different systems respond to flows, it is crucial to comprehend the influence played by swimming pattern. The tendency of organisms to navigate up or down the flow is termed rheotaxis. Early theoretical studies predicted a positive rheotactic response for puller-type spherical Janus micromotors. However, recent experimental studies have focused on pusher-type Janus particles, finding that they exhibit cross-stream migration in externally applied flows. To study the response to the flow of swimmers with a qualitatively different flow pattern, we introduce Cu@SiO2 micromotors that swim toward their catalytic cap. On the basis of experimental observations, and supported by flow field calculations using a model for self-electrophoresis, we hypothesize that they behave effectively as a puller-type system. We investigate the effect of externally imposed flow on these spherically symmetrical Cu@SiO2 active Janus colloids, and we indeed observe a steady upstream directional response. Through a simple squirmer model for a puller, we recover the major experimental observations. Additionally, the model predicts a jumping behavior for pullertype micromotors at high flow speeds. Performing additional experiments at high flow speeds, we capture this phenomenon, in which the particles roll with their swimming axes aligned to the shear plane, in addition to being dragged downstream by the fluid flow.

Automated summary

Fluid flow is common in microorganism habitats, and both biological and artificial microswimmers respond to flows influenced by chemical and physical factors. This study introduces Cu@SiO2 micromotors and investigates their behavior under externally applied flows. The micromotors exhibit puller-type characteristics and display jumping behavior at high flow speeds.