Pumpless, directional transport of gas bubbles on wettability-patterned diverging tracks

The manipulation of gas bubbles in an aqueous medium is useful for a variety of applications. Motion of bubbles on a submerged solid substrate is dominated by buoyancy, which renders bubble manipulation tasks difficult. A wettability-patterning technique that allows for continuous directional transport of gas bubbles on submerged flat surfaces, without any external source of energy, is presented. The design consists of a diverging superaerophilic track laid on a superaerophobic background. Gas bubbles are dispensed using a submerged nozzle at the narrow end of the superaerophilic track on the submerged substrate. The diverging nature of the track results in a Laplace pressure difference due to curvature variation of the bubble, which then propels it from the narrow to the wide end of the track. Spontaneous motion of gas bubbles on such tracks is demonstrated, and the forces responsible for such transport are identified. Scaling arguments for the nature of said forces are presented. The transport rates of gas bubbles are compared with a similar mode of droplet transport on superhydrophilic, diverging tracks laid on an open-air surface, and the salient differences are highlighted.