Self-Propulsion and Active Motion of Janus Ellipsoids

The propulsion of platinum-coated polystyrene prolate ellipsoids, as generated by catalytic decomposition of hydrogen peroxide, is characterized by direct visualization of the trajectories of the active particles. These Janus ellipsoids were fabricated by stretching micron-sized polystyrene spheres into different aspect ratios; half of the particle is then capped lengthwise along the ellipsoid's major axis, with platinum deposition. These particles exhibit complex dynamical trajectories in aqueous solutions of hydrogen peroxide of concentration in the range of 2-8% (w/v). In this range, a transition from three-dimensional passive Brownian motion to two-dimensional active motion is observed as the hydrogen peroxide concentration is increased. This transition from passive to active motion is complete by 4% (w/v) hydrogen peroxide. We quantify the effect of particle aspect ratio on the mean-squared displacement and mean-squared angular displacement at the highest hydrogen peroxide concentration. The two-dimensional trajectories of the individual particles were grouped into three categories for dynamical analysis. In the first category, ballistic ellipsoids translate at least 5 times more than purely diffusive ellipsoids at the characteristic time scale of rotational diffusion. In the second category, spinning ellipsoids move only short distances with a dominant rotation about the minor axis; this rotation persists for many revolutions. A third category captures trajectories that include both significant translation and rotation. We consider the physical origins of the observed categories of motion and extract the forces and torques generated by the catalytically generated propulsion as a function of aspect ratio. The particle velocity, and therefore the active force, increases with the aspect ratio.