Researchers aboard the International Space Station (ISS) analyzed how water droplets merge in cosmic microgravity.
Typically, water droplets are portions of liquid with a diameter of a couple of millimeters, above this volume, the drop loses its shape, given the surface tension. However, in space, there is almost no gravitational force, so droplets can be much larger.
“NASA astronauts Kathleen Rubins and Michael Hopkins would deposit a single drop of desired size at a central location on the surface. This drop is near, but not touching, a small porthole pre-drilled into the surface," said Josh McCraney of Cornell University in a press release. "The astronaut then injected water through the porthole, which collects and essentially grows an adjacent drop. Injection continues until the two drops touch, at which point, they coalesce."
The experiments were designed to test the Davis-Hawking model, a simple mathematical way to simulate droplets. If the water droplet is on the surface, part of it touches the air and creates an interface, and the part in contact with the surface forms a contact line. The Davis-Hawking model describes the equation for the contact line. Experimental results confirmed and expanded the parameter space of the Davis-Hawking model.