How advances in low-g plumbing enable space exploration.

In many ways, plumbing is essential to life support. In fact, the advance of humankind on Earth is directly linked to the advance of clean, healthy, reliable plumbing solutions. Shouldn't this also be true for the advancement of humankind in space? Unfortunately, the reliability of even the simplest plumbing element aboard spacecraft is rarely that of its terrestrial counterpart. This state of affairs is due entirely to the near-weightless "low-g" state of orbiting and coast spacecraft. But the combined passive capillary effects of surface tension, wetting, and system geometry in space can be exploited to replace the passive role of gravity on earth, and thus achieve similar outcomes there. In this paper, we review a selection of experiments conducted in low-g environments (i.e., ISS and drop towers) that focus on capillary fluidic phenomena. The results of each experiment are highly applicable to subsequent advances in spacecraft plumbing. With examples ranging from spurious droplet ejections to passive bubble coalescence, to droplet bouncing, to complex container wicking, we show how simple low-g demonstrations can lead to significant reliability improvements in practical passive plumbing processes from pipetting to liquid-gas separations, to wastewater transport, to drinking in space.

Capillary channel flow experiments aboard the International Space Station.

In the near-weightless environment of orbiting spacecraft capillary forces dominate interfacial flow phenomena over unearthly large length scales. In current experiments aboard the International Space Station, partially open channels are being investigated to determine critical flow rate-limiting conditions above which the free surface collapses ingesting bubbles. Without the natural passive phase separating qualities of buoyancy, such ingested bubbles can in turn wreak havoc on the fluid transport systems of spacecraft. The flow channels under investigation represent geometric families of conduits with applications to liquid propellant acquisition, thermal fluids circulation, and water processing for life support. Present and near future experiments focus on transient phenomena and conduit asymmetries allowing capillary forces to replace the role of gravity to perform passive phase separations. Terrestrial applications are noted where enhanced transport via direct liquid-gas contact is desired.