Confluent droplets are prominent in stormwater runoff from building facades, cell-cell interaction and cell attachment to biological tissues, condensing steam for power generation, etc. While knowledge of the fluid viscosity and surface tension is sufficient to describe the dynamics of free-floating confluent droplets, the solid-liquid-gas contact line the dynamics of sessile droplets.

In a new study, scientists from Cornell University and Clemson University designed and analyzed droplet experiments on the International Space Station.

On a small scale, the droplet dynamics are too fast to observe. Hence the ISS. The lower gravity in space means the team can examine larger droplets, from a few millimeters in diameter to 10 times that length.

The scientists sent four different surfaces to the ISS and installed them on a lab bench, each with a different roughness property. The spreading and merging of the droplets were recorded by cameras.

Centimeter-scale droplets fuse during an experiment on the International Space Station. CREDIT: Josh McCraney

Josh McCraney of Cornell University said: “NASA astronauts Kathleen Rubins and Michael Hopkins deposited a single droplet of the desired size at a central spot on the surface. This droplet is near, but does not touch, a small porthole pre-drilled into the surface. The astronaut then injected water through the porthole, which collects and essentially grows an adjacent droplet. Injection continues until the two droplets touch, after which they merge.

The Davis-Hocking model, a simple method to simulate droplets, was the subject of the studies. A water droplet resting on a surface has a part that touches the air, creating an interface, and a part that contacts the surface, creating an edge or line of contact. The equation for the contact line is described by the Davis-Hocking model. The parameter space of the Davis-Hocking model was enlarged and confirmed by the experimental findings.

To successfully conduct his research aboard the ISS, the late Professor Paul Steen of Cornell University had written grants, gone to collaborators in several countries, Ph.D. students, and scrupulously reviewed relevant terrestrial studies. Sadly, Steen died just a few months before his experiments began.

McCraney said: “While it is tragic that he is not here to see the results, we hope this work makes him and his family proud.”

Magazine reference:

  1. J. McCraney, J. Ludwicki, J. Bostwick, S. Daniel, and P. Steen. Coalescence-induced droplet dispersal: experiments aboard the International Space Station. Physics of Fluids. DOI: 10.1063/5.0125279