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Passive thermal management is of interest in cooling of electronics and avionics in terrestrial and reduced gravity environments. This paper describes the use of microscale asymmetric surface patterns, or ratchets, to generate preferential fluid motion during phase change. The asymmetric patterns take the form of an array of ratchet structures. Preferentially directed bubble growth is demonstrated for boiling on surfaces with such ratchets augmented with re-entrant cavities to produce nucleation at preferred sites. During pool boiling in FC-72, the asymmetric geometry of microstructures causes bubbles to grow normal to the sloped surface rather than in a vertical direction, resulting in a net motion in a preferential direction. Bubble growth from the re-entrant cavities is studied using high-speed photography and image processing techniques. The concept of self-propulsion is extended to an open-ended channel configuration, wherein high-speed videos that document preferential motion of vapor slugs with velocities in the range of several mm/s are presented. Liquid motion is explained using a semi-empirical force balance. 

Passive thermal management is of interest in cooling of electronics and avionics in terrestrial and reduced gravity environments. This paper describes the use of microscale asymmetric surface patterns, or ratchets, to generate preferential fluid motion during phase change. The asymmetric patterns take the form of an array of ratchet structures. Preferentially directed bubble growth is demonstrated for boiling on surfaces with such ratchets augmented with re-entrant cavities to produce nucleation at preferred sites. During pool boiling in FC-72, the asymmetric geometry of microstructures causes bubbles to grow normal to the sloped surface rather than in a vertical direction, resulting in a net motion in a preferential direction. Bubble growth from the re-entrant cavities is studied using high-speed photography and image processing techniques. The concept of self-propulsion is extended to an open-ended channel configuration, wherein high-speed videos that document preferential motion of vapor slugs with velocities in the range of several mm/s are presented. Liquid motion is explained using a semi-empirical force balance.