Asymmetric Sawtooth and Cavity-Enhanced Nucleation-Driven Transport (ASCENT) Experiment aboard the International Space Station – Microgravity Outcomes

Several forces exert an influence on two-phase bubble dynamics under terrestrial conditions, chief among these being those due to buoyancy and surface tension. Under microgravity conditions, the absence of buoyancy forces disrupts bubble dynamics preventing bubbles from detaching from surfaces. The stagnant bubbles form a large vapor mass attached to the surface, leading to a considerable rise in surface temperature. In this study, a mesoscale-engineered surface in the form of saw-toothed structures has been produced that can provide access to liquid pockets across the troughs of the microstructure in microgravity and terrestrial adverse-gravity orientation. The surface is built with intentional nucleation sites that support consistent vapor germination in both environments. Following terrestrial studies on various surface morphologies and under varying degrees of sub cooling, experiments were conducted onboard the International Space Station. The test chambers were square cross-sectioned glass ampoules with deposited thin film nichrome heaters. NASA's implementation partner developed the test and instrumentation hardware to conform to stringent flight requirements. The experimental investigation is titled Asymmetric Sawtooth and Cavity-Enhanced Nucleation-driven Transport (ASCENT) and was conducted in the Pore Formation and Mobility Investigation (PFMI) furnace. The paper discusses high- speed data obtained on vapor motion in microgravity and terrestrial downward-facing environments, providing insight into the differences between a flat surface and the microstructure. The images suggest the existence of a mobility diameter in microgravity, which enables favorable motion across the microstructure in both lateral directions. This mobility contrasts the slug mobility actuated by the sawtooth in the downward-facing terrestrial heater, where changes in the interfacial radius of curvature produce a net lateral motion in the direction of the long slope.