How advances in low-g plumbing enable space exploration

Abstract

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.

Fig. 1. The Capillary Beverage experiment.

a Image of the space cup (left), space cup demitasse (right), and stands, b solid model with labels, c sequence of tapering sections, and d water surface profiles (black lines) during continuous drinking by astronaut S. Kelly on ISS in time t with a large arrow indicating average liquid flow rate and direction.

Fig. 2. The Capillary Beverage experiment “science”.

a Space cup with various beverages: bubble-free water, limeade, Kona Black coffee with small bubbles and undissolved coffee particles, and peach-mango smoothie. b NASA astronauts S. Kelly, K. Lindgren, and JAXA astronaut K. Yui on ISS.

Fig. 3. Espresso in the demitasse Space Cup.

a Italian ESA astronaut S. Cristoforetti drinking ISSpresso espresso from demitasse Space Cup in the cupola of the ISS. b Magnified view of espresso located at cup lip with crema distributed throughout (courtesy NASA).

Fig. 4. Pipetting with well plates on ISS.

a German ESA astronaut A. Gerst pipetting in an open cabin of ISS in support of Protein Crystal Growth Experiments. b Gerst conducting STCE pipetting and stability tests on ISS using commercial well plates and low-g custom 3D printed capillary fluidic wells consisting of a tapered cone, rhombus, and “space cup” geometries (left to right with green-dyed water). Images of c vertical and d oblique tests. All well plates performed adequately with the capillary fluidic wells demonstrating greater fluid control and stability. Though not observed by eye, it is expected that nearly all pipette withdrawals resulted in spurious satellite droplet ejections, the majority of which are readily captured by local fan airflow identified in b.

Fig. 5. Filament rupture and droplet ejection during low-g pipetting from well plates.

a Low-g drop tower demonstrations of distinct liquid ejections from rupturing rivulets following pipette withdrawal: (top to bottom) no “observed” ejection, “Satellite” droplet ejection, “Mother” droplet, “Inertial Ligament,” and large Inertial Ligament ejection. b ESA astronaut A. Gerst pipetting aboard the ISS with no visible evidence of droplet ejection but magnified slow-motion observations reveal satellite droplet ejection (barely visible streaks within the white dashed circles) during nearly every pipette tip withdrawal event.

Fig. 6. Low-g rivulet rupture and automated test apparatus.

a Approximately 150 mL blob of Nickelodeon Slime was stretched into a rivulet between two ping pong paddles at velocity U_o ≈ 1 m/s by Italian ESA astronaut L. Parmitano. The length of the rivulet before rupture is >1 m and the five mother droplet diameters are ≈ 3 cm. b Drop tower and ISS data regime map of low-g “aqueous” rivulet rupture in terms of dimensionless inertia We_o^1/2 and viscous resistance Ca_o. The Slime data resides in the low inertial domain Su^1/2 << 1 with values of Ca_o at least three orders of magnitude higher than values attained on earth. c Automated pipetting, sample preparation, and sequencing experiment rig for high-rate rivulet rupture drop tower investigations and ejected droplet mitigation methods development with the magnified device shown in d red dashed region (courtesy IRPI LLC).

Fig. 7. Simple removal of a Petri dish lid during a drop tower test.

a Low-g equilibrium configuration, b, c lid removal produces films and rivulets that rupture producing hundreds of satellite droplets in d, e. The large rivulet at right in e again ruptures producing enormous spurious droplets.

Fig. 8. Selection of Capillary Flow Experiment (CFE) test sections and ISS results.

a Sketches of fluid bearing chambers for ISS capillary fluidics experiments CFE and CFE-2 to approximate scale with dimensions in cm: moving Contact Line dynamics and capillary stability (CL), critical geometric wetting in an elliptical cylindrical container with pivoting Vane with Gap between wall and vane (VG), and ullage migration due to Interior Corner Flow (ICF). b Flight images of static interface configurations in VG1 demonstrating critical geometric wetting and dynamic images in c ICF1, d ICF2, and e ICF8 demonstrating ullage migration with passive bubble phase separations.

Fig. 9. The Capillary Channel Flow (CCF-EU2) experiment with sample data.

a Solid model of CCF with an open wedge test cell circled in red. b Simplified schematic of open wedge channel depicting passive low-g capillary separation mechanism of a single bubble. c Sample bubble phase regime map as function of liquid Q_l and gas flow rate Q_g with single and merged bubble regimes identified with either 0% or ~100% passive bubble separations.

Fig. 10. Droplets ejected from low-g bubble bursts (identified by arrows or tracks).

a Bursting CO₂ bubbles from a dissolving antacid tablet eject droplets from a 65 mL water blob suspended from a wire hoop by US astronaut D. Pettit on ISS. b Similarly ejected droplets from similarly bursting CO₂ bubbles in the Space Cup which confines the bubbles away from the primary capillary flow interior corner region of the cup as demonstrated by K. Lindgren. c Ejected droplet(s) from bursting bubbles as Canadian astronaut C. Hadfield wrings out a partially water-saturated cloth. The ejected droplet identified impacts the camera lens. d Single image of a ‘large’ ejection following a bubble burst within an inertial vertical up flow of effervescing sparkling water during a low-g drop tower test. An ~ 2 cm wide FOV of the similar test as shown in d is shown in e, f, where image overlays at 2125 fps are presented e before and f after the step reduction in buoyancy during a drop tower test.

Fig. 11. Demonstrations of super-hydrophobic surfaces by ISS crew.

ISS astronauts a S. Kelly, b T. Peake, and c N. Hague and C. Koch demonstrate large length scale non-wetting phenomena with super-hydrophobic paddles and water blobs of ~ 10, 30, and 250 mL, respectively (see white arrows).

Fig. 12. Drop tower test demonstrations of large length scale super-hydrophobic (SH) capillary phenomena.

a Time overlay of 1 mL urine ersatz puddle jump rebounding between non-parallel plates. b Simultaneous drop tower tests of two 6 mL/s oblique jet impacts of ersatz urine on SH and H (hydrophobic, non-wetting) substrates illustrate the ease with which SH substrates repel the jet (courtesy IRPI LLC). The simply hydrophobic (H) surface does not yield rebound, but a wall-bound rivulet. We note also that such rebound phenomena do not occur at 1-g_o for the flow rates and properties of typical human urination. c–g The spontaneous ejection of a 5 mL droplet from a water puddle between SH tilted plates at 5°: c static 1-g_o interface, b–f capillarity-driven acceleration, and g non-oscillatory ejection at steady velocity U_o ≈ 10 cm/s.