Flickering droplets in space confirm late professor’s theory

At a time when astronomers around the world are reveling in new views of the distant cosmos, an experiment on the International Space Station has given Cornell researchers new insight into something a little closer to home: water. .

Specifically, the space station’s microgravity environment has illuminated the ways in which water droplets oscillate and propagate through solid surfaces— knowledge that could have very real-world applications in 3D printing, spray cooling, and manufacturing and coating operations.

The research team’s paper, “Oscillations of Drops with Mobile Contact Lines on the International Space Station: Elucidation of Terrestrial Inertial Droplet Spreading,” published August 16 in Physical examination letters. The lead author is Joshua McCraney, Ph.D.

The experiment and its results, while successful, are also bittersweet. The paper’s co-lead author, Paul Steen, Maxwell M. Upson Professor at the College of Engineering’s Smith School of Chemical and Biomolecular Engineering, died in September 2020, just before the experiment was performed.

“It’s sad that Paul couldn’t see the experiments launch into space,” said co-lead author Susan Daniel, Fred H. Rhodes professor at the Smith School of Chemical and Biomolecular Engineering and longtime collaborator date of Steen. “We hope we did well in the end, and the paper we produced from the work will make him proud.”

Daniel began collaborating with Steen shortly after he arrived at Cornell as an assistant professor in 2007. While his current research focuses on the biological interface of the coronavirus, his postgraduate work focused on chemical interfaces and fluid mechanics– an area in which Steen advanced a number of theoretical predictions based on how droplets resonate when subjected to vibrations. The two researchers immediately connected.

“He knew the theory and made predictions, and I knew how to run the experiments to test them,” Daniel said. “Basically, from the time I arrived here in 2007 until his passing, we worked to try to understand how liquids and surfaces interact with each other, and how the line of contact at the interface between they behave under different conditions.

Their collaboration resulted in a “photo album” of dozens of possible shapes that an oscillating drop of water can take. Steen then expanded on this project by cataloging the energy states of droplets as evidenced by these resonant shapes, organizing them into a “periodic table” classification.

In 2016, Steen and Daniel received a four-year grant from the National Science Foundation (NSF) and NASA’s Center for the Advancement of Science in Space to conduct fluid dynamics research aboard the US National Laboratory of the International Space Station.

Space is an ideal place to study the behavior of fluids due to the drastic reduction in gravity, which on the ISS is about one millionth of its Earth level. This means that fluid-surface interactions that are so small and fast on Earth that they are practically invisible can be, in space, almost 10 times larger – from microns to centimeters – and their duration slows down by almost 30 time.

“It’s harder to study these falling motions, experimentally and fundamentally, when you have gravity in your way,” Daniel said.

Steen and Daniel selected a few resonance shapes from their photo album that they wanted to explore in detail, focusing on how a drop of water’s contact line – or outer edge – glides from back and forth on a surface, causing the liquid to spread. , a phenomenon that can be controlled by varying the vibration frequencies.

The team prepared meticulous instructions for the astronauts to follow, compressing four years of planning into a minutes-long experience in which every second was tightly choreographed.

As the researchers monitored and provided real-time information on the ground, the astronauts deposited 10ml water droplets via a syringe onto nine different hydrophobic surfaces with varying degrees of roughness. They also forced pairs of droplets to merge and placed droplets on an oscillator and tuned its vibrations to achieve the targeted resonance shapes. The oscillating and agitating motions of the water droplets were filmed, and the researchers spent the next year analyzing the data.

This analysis ultimately confirmed Steen’s theories of how the density and surface tension of a liquid control the mobility of the contact line, overcoming the roughness of a surface.

Daniel credits co-author Joshua Bostwick, Ph.D., a former student of Steen and now an associate professor in the Stanzione Collaboration at Clemson University, for ensuring that the results of the experiment match the Steen’s theoretical predictions.

“Josh was able to continue the theoretical aspect of this work in Paul’s absence, which I wasn’t ready to do. It was nice to see him join the team and help us make sure we were able to extract everything we could from the data that we collected,” Daniel said. “Now we can basically use the theory that Paul created to make predictions, for example, in processes where you spray droplets on surfaces, or in 3D printing, or when liquids spread very quickly over a surface.”

Vanessa Kern, Ph.D. was also a co-author of the article.