Autonomous microscopic agents moving through confined, liquid-filled spaces are envisioned as a key component of future lab-on-a-chip and drug delivery systems. Chemically active Janus particles offer a realization of such agents. Depending on the system, various self-propulsion mechanisms emerge, such as bubble propulsion, self-electrophoresis or self-diffusiophoresis. Here, we discuss the self-propulsion of a Janus spheroidal particle driven by self-diffusiophoresis. First, we describe how the swimming velocity depends on the particle's aspect ratio and on the catalyst coverage. Next we analise how such active particles can be used as carriers of micro-cargo, and show that the velocity of the carrier-cargo composite strongly depends on the relative orientation of the link between the active and passive particles.
Near a hard wall, a Janus active particle reveals a very interesting behavior, including novel sliding and hovering steady states. The sliding steady state provides a starting point to engineer a stable and predictable motion of microswimmers. For example, at topographically patterned walls, novel states of guided motion along the edges of the geometrical patterns can emerge. We also predict that the trajectories of chemical microswimmers can be efficiently controlled by using chemically patterned walls. The induced chemi-osmotic flows at the wall can cause particles to either “dock” at the chemical step or to robustly follow a thin chemical stripe.
Finally, we demonstrate that platinum microparticles move spontaneously in solutions of hydrogen peroxide and that their motions can be rationally designed by controlling particle shape. The observed relationships between particle shape and motion provide evidence for a self-electrophoretic propulsion mechanism, whereby anodic oxidation and cathodic reduction occur at different rates at different locations on the particle surface.