Large Length Scale Capillary Fluidics: From Jumping Bubbles to Drinking in Space

In orbit, finding the bottom of your coffee cup is a non-trivial task. Subtle forces often masked by gravity influence the containment and transport of fluids aboard spacecraft, often in surprising non-intuitive ways. Terrestrial experience with capillary forces is typically relegated to the micro-scale, but engineering community exposure to large length scale capillary fluidics critical to spacecraft fluid management design is low indeed. Low-cost drop towers and fast-to-flight International Space Station (ISS) experiments are increasing designer exposure to this fresh field of study. This work first provides a wide variety of drop tower tests that demonstrate fundamental and applied capillary fluidics phenomena related to liquid droplets and gas bubbles. New observations in droplet auto-ejection, droplet combustion, forced jet combustion, puddle jumping, bubble jumping, and passive phase separation are presented. We also present the Capillary Beverage Experiment on ISS as a fun and enlightening application of capillary fluidics where containment and passive control of poorly wetting aqueous capillary systems is observed. Astronauts are able to smell their coffee from the open stable container while still drinking in an Earth-like manner with the role of gravity replaced by the combined effects of surface tension, wetting, and special container geometry. The design, manufacture, low-g demonstrations, and quantitative performance of the Space Cups are highlighted. Comparisons of numerical simulations, drop tower experiments, and ISS experiments testify to the prospects of new no-moving-parts capillary solutions for certain water-based life support operations aboard spacecraft