Analysing the merging of water droplets on the International Space Station

Researchers from Cornell University and Clemson University designed and analysed water droplet experiments that were done on the International Space Station (ISS).

Understanding how water droplets spread and coalesce is essential for scenarios in everyday life, such as raindrops falling off cars, planes, and roofs, and for applications in energy generation, aerospace engineering, and microscale cell adhesion. However, these phenomena are difficult to model and challenging to observe experimentally.

The research, ‘Coalescence-induced droplet spreading: Experiments aboard the International Space Station,’ aimed to understand the movements of these droplets and was published in Physics of Fluids.

How gravity enabled the droplets to be observed

Water droplets usually appear as small spherical caps of water because their surface tension exceeds gravity.

“If the drops get much larger, they begin to lose their spherical shape, and gravity squishes them into something more like puddles,” said Josh McCraney from Cornell University.

“If we want to analyse drops on Earth, we need to do it at a very small scale.”

However, at small scales, droplet dynamics are too fast to observe. This is where the ISS played a major role. The lower gravity in space means the team could investigate larger droplets, moving from a couple of millimetres in diameter to ten times that length.

The researchers sent four different surfaces with various roughness properties to the ISS, where they were mounted to a lab table. Cameras recorded the water droplets as they spread and merged.

“NASA astronauts, Kathleen Rubins and Michael Hopkins, would deposit a single drop of the desired size at a central location on the surface,” McCraney explained.

“This drop is near, but not touching, a small porthole pre-drilled into the surface. The astronauts 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.”

Testing the Davis-Hocking model

The experiments aimed to test the Davis-Hocking model, a simple way to simulate water droplets. If a droplet of water sits on a surface, part of it touches the air and creates an interface, while the section in contact with the surface forms an edge or contact line.

The Davis-Hocking model describes the equation for the contact line. The experimental results confirmed and expanded the parameter space of the Davis-Hocking model.

As the original principal investigator of the project, the late professor Paul Steen of Cornell University had written grants, travelled to collaborators worldwide, trained doctoral students, and meticulously analysed related terrestrial studies, all with the desire to see his work successfully conducted aboard the ISS. Tragically, Steen died only months before his experiments launched.

McCraney concluded: “While it’s tragic he isn’t here to see the results, we hope this work makes him and his family proud.”

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