Self-propulsion of aluminum particle-coated Janus droplet in alkaline solution.

Janus droplet motion powered by gas bubbles is studied in this paper. The Janus droplets were fabricated by partially covering one side of an oil droplet with aluminum particles under gravity effect. By placing the Janus droplet in an alkaline solution, the reaction between the Al particles and OH- generates H2 gas, which emits from the particle-coated side of the droplet as bubbles. Hence, the droplet is propelled to move in the opposite direction. In this research, the effects of several factors, time, pH, particle coverage and surfactant, on the motion of the Janus droplet were studied. The experimental results indicate that as time progresses, the droplet motion experiences three periods: initial development stage, stable stage and decline stage. The speed of the droplet increases with the pH value and the particle coverage. Comparing with the other three surfactants, Tween 20, CTAB and SDS, the surfactant Triton X-100 is the best choice for generating the spontaneous motion of the Janus droplets. Furthermore, the directional motion of the Janus droplets was examined, and the results show that the controllable transportation of the Janus droplets can be accomplished by externally applied DC electric field. The spontaneous motion of the droplets offers great promise in chemical and biological applications.