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.
The breakup and rupture of liquid bridges, thin films, bubbles, droplets, rivulets, and jets can produce satellite droplets that are subsequently ejected into their surrounding environment. For example, when any solid object is withdrawn from a liquid bath, the formation of an ever-thinning columnar liquid bridge eventually ruptures along the axis of the bridge. When rupture occurs under typical pipetting conditions the dynamics governing the rupture almost always produce at a minimum a satellite droplet. When these droplets occur they are often too small and too fast to be observed by the human eye. In a terrestrial environment they are of little concern due to the gravitational force imperceptibly returning these droplets back to the bulk fluid. This is not the case for low-g environments where activities such as pipetting creates satellite droplets that are ejected far away from the source fluid creating a risk of contamination within the surrounding working environments. In this work we demonstrate a variety of droplet ejections for the application of pipetting in space and highlight how in a low-g environment such dynamics depend on system geometries, fluid properties, wettability, and withdrawal rate. A drop tower data set is collected in support of a regime map organized by withdrawal Weber and Capillary numbers that highlight when different fluid ejection types are to be expected. Mitigation techniques are presented as a design guide for further applications aboard spacecraft.
The major factor influencing the behavior of microbes growing in liquids in space is microgravity. We recently measured the transcriptomic response of the Gram-positive bacterium Bacillus subtilis to the microgravity environment inside the International Space Station (ISS) in spaceflight hardware called Biological Research in Canisters-Petri Dish Fixation Units (BRIC-PDFUs). In two separate experiments in the ISS, dubbed BRIC-21 and BRIC-23, we grew multiple replicates of the same B. subtilis strain in the same hardware, growth medium, and temperature with matching ground control samples (npj Micrograv. 5:1.2019, doi: 10.1038/s41526-018-0061-0). In both experiments we observed similar responses of the transcriptome to spaceflight. However, we also noted that the liquid cultures assumed a different configuration in microgravity (a toroidal shape) compared with the ground control samples (a flat disc shape), leading us to question whether the transcriptome differences we observed were a direct result of microgravity, or a secondary result of the different liquid geometries of the samples affecting, for example, oxygen availability. To mitigate the influence of microgravity on liquid geometry in BRIC canisters, we have designed an insert to replace the standard 60-mm Petri dish in BRIC-PDFU or BRIC-LED sample compartments. In this design, liquid cultures are expected to assume a more disk-like configuration regardless of gravity or its absence. We have: (i) constructed a prototype device by 3D printing; (ii) evaluated different starting materials, treatments, and coatings for their wettability (i.e., hydrophilicity) using contact angle measurements; (iii) confirmed that the device performs as designed by drop-tower testing and; (iv) performed material biocompatibility studies using liquid cultures of Bacillus subtilis and Staphylococcus aureus bacteria. Future microgravity testing of the device in the ISS is planned.
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