In particular, the space station’s microgravity environment shed light on the ways water droplets oscillate and spread on solid surfaces — knowledge that could have very Earthly applications in 3D printing, spray cooling, and manufacturing and coating operations. The team’s paper, “Drop Oscillations with Moving Contact Lines on the International Space Station: Elucidating Ground-Inertial Drop Spreading,” was published Aug. 16 in Physical Review Letters. Lead author is Joshua McCraney, MS ’19, Ph.D. ’21. The experiment and its findings, while successful, are also bittersweet. The paper’s co-senior author Paul Steen, the Maxwell M. Upson Professor in the College of Engineering’s Smith School of Chemical and Biomolecular Engineering, died in September 2020, shortly before the experiment was conducted. “It’s unfortunate that Paul didn’t get to see the experiments launch into space,” said co-senior author Susan Daniel, the Fred H. Rhodes Professor in the Smith School of Chemical and Biomolecular Engineering and Steen’s longtime collaborator. “We hope we did right by him in the end and that the paper we created from the project would make him proud.” Daniel began working with Steen shortly after she first came to Cornell as an assistant professor in 2007. While her current research focuses on the biological interface of the coronavirus, her graduate work was on chemical interfaces and fluid mechanics — a field in which Steen advanced a series of theoretical predictions based on how droplets resonate when subjected to vibrations. The two researchers connected immediately. “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 got here in 2007 until he passed away, we worked to understand how liquids and surfaces interact with each other and how the contact line behaves at the interface between them under different conditions.” Their collaboration resulted in a “photo album” of the dozens of possible shapes a wobbly drop of water can take. Steen later expanded on this work by recording the energy states of the droplets as evidenced by these sound patterns, 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 at the US National Laboratory on the International Space Station. Space is an ideal place to study the behavior of fluids because of the radical reduction in gravity, which in the ISS is about one millionth of its ground level. This means that liquid-surface interactions that are so small-scale and fast on Earth that they are practically invisible can be, in space, nearly 10 times larger — from microns to centimeters — and their durations are slowed by nearly 30 times. “It’s harder to study these falling motions, experimentally and basically, when you have gravity in your way,” Daniel said. Steen and Daniel selected a few tuning shapes from their photo album that they wanted to explore in detail, focusing on how a water drop’s contact line — or outer edge — slides back and forth across a surface, leading the way by which the fluid will spread, a phenomenon that can be controlled by varying vibration frequencies. The team prepared meticulous instructions for the astronauts to follow, squeezing four years of planning into a multi-minute experiment in which every second was tightly choreographed. With researchers monitoring and providing real-time feedback on the ground, the astronauts deposited 10 mL water droplets via a syringe onto nine different hydrophobic surfaces of varying degrees of roughness. They also forced pairs of droplets to join together and placed droplets in an oscillator and tuned its vibrations to achieve the targeted tuning patterns. The wobbling and flickering movements of the water droplets were videotaped, and the researchers spent the next year analyzing the data. This analysis finally confirmed Steen’s theories about how the density and surface tension of a liquid control contact line mobility, overcoming the roughness of a surface. Daniel credits co-author Joshua Bostwick, Ph.D. ’11, a former student of Steen’s and now Stanzione Collaboration Associate Professor at Clemson University, ensuring that the experiment’s results matched Steen’s theoretical predictions. “Josh was able to continue the theoretical side of this work in Paul’s absence, which I wasn’t ready to do. we’re extracting what we can from the data we’ve collected,” Daniel said. “Now we can actually use the theory that Paul created to make predictions, for example, in processes where you spray droplets on surfaces, or in 3D printing, or where liquids spread across a surface very quickly.” Vanessa Kern, Ph.D. ’20, was also a co-author of the paper. The research was supported by NSF and NASA’s Center for the Advancement of Science in Space.
