Feeding astronauts on deep space missions with fresh produce will require robust plant cultivation methods. However, watering plants in microgravity presents unique problems. Without gravity, bubbles don't rise and droplets don't fall, leading to unstable jets, trapped gases, and unpredictable fluid dynamics that traditional Earth-based irrigation systems can't handle.

To tackle these challenges, NASA launched the Plant Water Management (PWM) experiment series aboard the International Space Station in 2021. These experiments are ongoing and are also providing insights applicable to other spacecraft systems such as fuel handling, HVAC, and waste collection.

The latest iteration, PWM-5 and PWM-6, features three test units containing variable-speed pumps, tubing, valves, syringes, and either serial or parallel hydroponic channels. This configuration allows researchers to examine a wider set of variables, including flow rates, fill levels, gas-liquid separation, and various plant root structures.

Made primarily of 3-D printed flight-certified materials, the hardware is assembled by astronauts aboard the station. Crews configure and operate the equipment on an open cabin workbench, capturing data via a single HD camera while coordinating with ground-based researchers.

The PWM hardware tests multiple critical functionalities: hydroponic and ebb and flow modes, system priming and draining, channel operations, bubble management, and steps for clean plant insertion and removal. These detailed procedures help ensure stable and repeatable fluid dynamics despite the absence of gravity.

PWM-5 and -6 trials have proven that plant-watering methods using serial and parallel channel configurations can achieve effective single- and two-phase flow rates across various engineered plant root models. The systems successfully rely on surface tension and geometric design to manage gas-liquid separation, taking over gravity's role in fluid behavior.

For instance, the bubble separator eliminates all oxygen bubbles, enabling pure liquid flow. The water trap captures stray liquid, and the bubble diverter routes bubbles through a geometric trap where they coalesce and exit-making for a triple-layered, 100% passive gas-liquid management system.

These advances mean the PWM technologies offer off-the-shelf solutions for microgravity farming, even with the difficult surface properties of water-based nutrient solutions. Though many root types have been simulated, researchers note the next critical step will be testing with live growing plants in these passive systems.

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