At a Glance
- Liquids behave much differently in microgravity than they do on Earth, and examining liquid motion on the ISS allows scientists to study fluid dynamics in ways not possible on the ground.
- An upcoming ISS National Lab investigation co-sponsored by the National Science Foundation aims to study the movement of liquid droplets across a solid surface to gain a better understanding of inertial spreading.
- Results from this investigation could help improve many terrestrial processes that involve inertial spreading, with important applications in the manufacturing, agricultural, and medical industries.
- Members of the research team recently published a journal article in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) discussing their research, including their upcoming ISS National Lab investigation.
Although it is mesmerizing to watch large bubbles of liquid float around on the International Space Station (ISS), looking at liquid motion in microgravity is about more than just aesthetics—it has significant scientific benefits as well. Studying liquid motion onboard the ISS National Lab provides valuable insights into fluid dynamics that have important applications back on Earth.
In microgravity, liquids held together by surface tension can achieve sizes much larger than liquids on the ground. Additionally, when these liquids move, the motions are much slower than on the ground. On the ISS National Lab, scientists can study liquid motions that are small-scale and fast on Earth in an environment where the motions occur on a larger scale and at a slower pace, making optical imaging of the motions easier.
A team of scientists led by Cornell University researcher Dr. Paul Steen is leveraging the microgravity environment on the ISS National Lab to study inertial spreading, or the movement of liquid across a solid surface in such a way that liquid inertia and surface tension are the dominant competitors. Inertial spreading is critically important in many applications on Earth, including several manufacturing, agricultural, medical, and other industrial processes—and a better understanding of fast liquid movement across a solid surface could lead to improvements in these processes.
The research team’s ISS National Lab investigation, which was co-sponsored by the National Science Foundation (NSF), is expected to launch later this year. Steen and his team recently described their research, including their upcoming investigation on the ISS National Lab, in a journal article in the Proceedings of the National Academy of Sciences of the United States of America (PNAS).
The ISS National Lab partnered with NSF for a joint fluid dynamics solicitation in which NSF provided $1.5 million to support five ISS National Lab projects awarded in 2016, including Steen’s investigation on inertial spreading. The ISS National Lab has collaborated with NSF on multiple solicitations in the areas of fluid dynamics, combustion, and tissue engineering.
Creating a Periodic Table of Droplet Motions
Liquid droplets in motion take on complex shapes in the form of waves. The different frequencies and shapes of these waves form a spectrum of liquid motions, and Steen and his team created a “wettability spectrometer” to determine how easily droplets with certain frequencies and shapes would move across a solid surface.
Steen and his team had previously developed an equation allowing them to predict the specific frequencies and shapes of droplet motions. The team then created a periodic table of droplet motions, similar to the periodic table of elements, in which droplet motions called “motion elements” are arranged in order from lower energy to higher energy and according to their shape symmetries.
In a droplet on a support surface, the point at which the liquid, the air, and support surface meet is known as the “contact line.” The angle between the droplet and the surface is called the “contact angle.” Some droplets remain stationary on a support surface while others are mobile.
In droplets that are fully immobile (do not move across the support surface), the contact line does not move—only the contact angle changes with the motion of the droplet. In droplets that are fully mobile (with no resistance from the support surface), the contact angle does not change, and the contact line moves across the surface as the droplet moves. However, most droplets fall somewhere between these two. These droplets are called partially mobile, meaning they move across the support surface, but with some resistance. In this case, both the contact angle changes and the contact line moves as the droplet moves.
Taking Liquid Motions to Microgravity
On the ground, the team has been able to observe the motion of immobile droplets and confirm several predicted frequencies and shapes. However, it is much more difficult to study partially mobile droplets because the droplets are so small and move so quickly that their movement is difficult to measure. This is why studying liquid motion on the ISS National Lab is beneficial—in microgravity, the research team can increase the size of the droplets, and the motion of the droplets is slower, making measurement easier.
The team’s ISS National Lab investigation will include two setups. The first will induce movement in a droplet of liquid on a surface using a platform that is vibrating up and down. The second will induce movement in two droplets of liquid on a surface using a third droplet to coalesce, or bring together, the two droplets. Video cameras on the ISS will record the motion of the droplets.
By examining droplet motion on the ISS, Steen and his team aim to measure droplet mobility to validate their predictions and gain a better understanding of inertial spreading. The team also hopes to observe several new motion elements that they have been unable to observe on the ground.
Applying Results to Earth-Based Processes
Knowledge gained from this investigation has many important applications back on Earth. For example, inertial spreading is a fundamental process involved in immersion lithography—an important technology in the manufacture of semiconductor chips that has enabled faster computer processors. Insights into inertial spreading resulting from this investigation could lead to improvements in immersion lithography that could help reduce the number of defects in semiconductors manufactured today.
For more information about the research being done by Steen and his team, view this article and video from Cornell University.