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The ability to separate liquid and gas phases in the absence of a gravitational acceleration has proven a challenge to engineers since the inception of space exploration. Due to our singular experience with terrestrial systems, artificial body forces are often imparted in multiphase fluid systems aboard spacecraft to reproduce the buoyancy effect. This approach tends to be inefficient, adding complexity, resources, and failure modes. Ever present in multiphase phenomena, the forces of surface tension can be exploited to aid passive phase separations where performance characteristics are determined solely by geometric design and system wettability. Said systems may be readily designed as demonstrated herein where a regulated bubbly flow is drawn through an open triangular sectioned duct. The bubbles passively migrate toward the free surface where they coalesce and leave the flow. The tests clearly show container aspect ratios required for passive phase separations for various liquid and gas flow rates. Preliminary data are presented as regime maps demarking complete phase separation. Long duration microgravity experiments are performed aboard the International Space Station. Supplementary experiments are conducted using a drop tower.

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