Space experiment could teach us how aerosols move in the atmosphere 

As a child, Jeffrey Moran was fascinated by outer space. Now, he is designing an experiment to be carried out on the International Space Station (ISS) in 2025. 

"I was obsessed with space as a kid,” said the mechanical engineering assistant professor. “The house I grew up in is still filled with drawings of Space Shuttles. Airplanes and spacecraft were a major reason I chose engineering when I got to college." 

When the National Science Foundation issued a call for proposals to conduct research projects on the ISS to benefit life on earth, Moran jumped at the chance and wrote a proposal that was successfully funded. His grant-winning experiment will examine the extent to which aerosols (small particles suspended in air) move through air in response to a temperature difference (meaning the air on one side is hotter than on the other). This phenomenon—migration of particles in response to temperature gradients—is known as thermophoresis. 

“It all started with a brainstorming session I had with a collaborator,” said Moran, describing the conversation he had with Purdue University colleague David Warsinger, who is a co-investigator on the NSF-funded project. “In contrast to most of my projects, which consider particles that move through liquids, we considered a simple question: what mechanisms could we use propel small particles through air?” 

“My lab focuses on developing self-propelled particles for applications like water treatment or drug delivery," said Moran. "It’s a new and exciting field, but it’s so far been restricted entirely to water environments. No one has tried to develop a swimmer that moves in air. That ties into climate change, because aerosols are everywhere in the atmosphere—both because of human activity, like burning fossil fuels, and because of natural events like volcanic eruptions—and we don’t have a solid understanding of the net effect aerosols will have on the climate.”  

Thermophoresis occurs in both liquids and gases, but it’s difficult to study in gases on earth because of the influence of gravity. Another challenge with studying particle migration in temperature gradients on earth is that the heated air tends to rise (for the same reason that hot air balloons rise), making it difficult to know exactly why the particles are moving. Doing the experiment in space allows scientists to run their tests with a minimal influence of gravity, to purely examine the effect of temperature on aerosols without creating air currents, which inevitably form when one tries to create a temperature difference in air because hot air tends to rise (which is the reason hot air balloons rise).  

“We’re going to send various aerosol samples into space, each in a specially designed cuvette, for the astronauts to test,” Moran explained. “There’s a microscope on the ISS, and the astronauts will place our cuvettes into an apparatus we’ve designed that applies heat and cold to opposite walls of the cuvette. The astronauts will then use the microscope to determine how fast the particles move towards hot or towards cold. We expect that aerosols made from different materials will respond differently to the temperature gradient, but nobody knows how. That’s what’s exciting about this experiment – no one has made these types of measurements before.” 

“For the last part of the project, we’re going to see whether some particles with asymmetric properties might generate the propulsive temperature difference on their own,” Moran continued. “These particles will have half of their surface coated in a metal. The other side is an insulating material,” Moran explained. “When we shine a light on them, the metal side efficiently absorbs the light and heats up relative to the insulating side. The hot hemisphere heats the air near it, and that creates a temperature difference in the surrounding air. This could help us understand whether odd-shaped aerosols in the atmosphere move on their own, without the need for a temperature difference.”  

Before the experiment can be carried out, Moran and his team need to determine the experiment’s parameters as well as the materials required.  

“This is very much a work in progress,” said Moran. “We're figuring out exactly which particles we want to send to the space station [based on] which materials matter the most to climate scientists [and] what the biggest question marks are.” 

He mentioned, among other examples, the possibility of experimenting with carbon soot.  

“Carbon soot is produced by burning fossil fuels, and it’s known to be harmful to the environment because it absorbs sunlight efficiently,” he said. “Another source of carbon soot, increasingly common in this era, is from rocket launches.”  
“It’s pretty well established that carbon soot overall intensifies warming,” he said. “It's black, so it tends to absorb sunlight efficiently. This leads to a net warming effect on the atmosphere, but it's not clear how much it moves in thermophoresis (how much it moves in temperature differences).” 

Moran looks forward to working with the National Aeronautics and Space Administration (NASA) to refine the plan. An early step in this project will be for Moran and his team to travel to Texas to examine a replica of the equipment available to scientists on the International Space Station. 

The College of Engineering and Computing will cover Moran’s progress as the project moves forward.