On the Lateral Motion of Bubbles Generated From Re-Entrant Cavities Located on Asymmetrically Structured Surfaces

This paper describes a novel concept of lateral motion of bubbles in pool boiling, which has the potential to be translated into a liquid propulsion system when used in a closed loop. The lateral motion of bubbles is achieved due to nucleation from cavities on an asymmetric saw-tooth profile created on a silicon surface. The surface modification involves etching a 3D sawtooth structure with a nominal angle of approximately 24° using gray-scale lithography. The downstream slope of each sawtooth structure features re-entrant cavity structures that act as controlled nucleation sites. The angle of the surface thus obtained causes a net imbalance of forces acting in concert on the flow field around the bubbles departing from the surface. The first part of the paper discusses the steps involved in fabricating such a heat sink with a saw-tooth structure augmented by re-entrant cavities. This is followed by description of the experimental facility used for studying the feasibility of the concept. High-speed photography in conjunction with bubble tracking is used to determine the bubble velocities. Results for a subcooled condition show substantial axial bubble velocities on the order of up to 68.5 cm/s near the cavities and a far-field velocity of up to 4 cm/s.