(427c) Selecting the Swimming Mechanisms of Colloidal Particles: Bubble Propulsion Vs. Self-Diffusiophoresis

Biological motors, such as F1-ATPase, myosin, kinesin, and bacterial flagellar, mostly rely on converting chemical fuels into mechanical motions. However, their complexity in structures and sensitivity to environmental conditions limited their applications. Therefore studies in synthetic micro-motors have thrived over the last decade. A wide range of propulsion mechanisms have been proposed such as the self-diffusiophoresis, bubble recoil, self-electrophoresis, magnetic field, the Marangoni effect, and enzymatic reactions. Among different types of propulsion strategies, the self-diffusiophoresis and bubble propulsion, using hydrogen peroxide as the fuel, have been widely studied. Although micro-tubular particles, when coated with platinum in their interior concave surfaces, can propel due to the formation and release of bubbles from one end (which should contributes to the constrained diffusion of oxygen molecules), the convex Janus particles usually do not generate any visible bubble. They move primarily due to the self-diffusiophoresis with quite small velocities.   

In this work, we use a simple bulk chemical deposition method to make platinum-polystyrene Janus dimers. Surprisingly, those convex particles are propelled by periodic growth and collapse of bubbles on the platinum-coated lobes. Furthermore, when we use the similar synthetic strategy to chemically deposit platinum on spheres that previously propelled with the self-diffusiophoresis principle, they are now propelled due to bubble propulsion. By comparing the platinum deposition method, surface coverage, and surface roughness, we confirm that both high catalytic activity and rough surface are necessary to tune the propulsion mode from self-diffusiophoresis to bubble propulsion. Compared with spheres, our Janus dimers, with combined geometric and interfacial anisotropy, also exhibit distinctive motions at the respective stages of bubble growth and collapse, which differ by 5-6 orders of magnitude in time. We have observed three types of motions from the hybrid dimers: linear, clockwise and counterclockwise motions, depending on the position of the bubble formation on the dimer. Interestingly, our Janus dimers exhibit opposite motion behavior at the bubble burst moment in linear and circular motions. Our study not only provides insight into the link between self-diffusiophoresis and bubble propulsion, but also reveals the intriguing impacts of the combined geometric and interfacial anisotropy on the self-propulsion of anisotropic particles.