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Free keywords:
Bubbles, Contact angle, Electrowetting, Oscillation, Hydrophobicity
Abstract:
Controlled wetting and de-wetting of surfaces is a primary mechanism used by beetles in nature such as the lady bird and the leaf beetle for underwater locomotion. Their adhesion to surfaces underwater is enabled through the attachment of bubbles entrapped in their setae covered legs. Locomotion, however, is done by applying mechanical forces on the bubbles in order to move, attach and detach them in a controlled manner. In synthetic conditions, however, when a bubble is bound to a surface, it is merely impossible to maneuver without the use of external stimuli. Thus, actuated wetting and de-wetting of surfaces remain challenges. Here, electrowetting on dielectrics (EWOD) is used for the manipulation of bubble-particle complexes on unpatterned surfaces. Bubbles nucleate on catalytic Janus disks adjacent to a hydrophobic surface. By changing the wettability of the surface through electrowetting the bubbles show a variety of reactions, depending on the shape and periodicity of the electric signal. Time resolved (µs) imaging of bubble radial oscillations reveals the possible mechanisms for lateral mobility of bubbles on a surface under electrowetting: bubble instability is induced when electric pulses are carefully adjusted. This instability is adjusted to control the surface bound bubble locomotion and described in terms of the change in surface energy. It is shown that a deterministic force applied normal can lead to a random walk of micrometer-sized bubbles by exploiting the phenomenon of contact angle hysteresis. Finally, bubble use in nature for underwater locomotion and the actuated bubble locomotion presented in this study are compared.
