Scientists at Northwestern University are taking their flourishing freeze-casting research, which may eventually enable manufacturing in space, to new heights: the International Space Station (ISS).
The 2021 Space Station project will come on the heels of another Northwestern experiment that NASA is launching into low-orbit via a small satellite in 2020.
Student researchers who comprise the SpaceICE team in the lab of principal investigator David Dunand, materials science and engineering, are finalizing plans for next year’s launch, as they enter the early stages of the 2021 mission to the ISS, where astronauts will monitor the experiments over a six-month timeframe.
Freeze-casting is a manufacturing methodology that years from now could free space explorers from expensive, time-consuming supply deliveries from Earth. In freeze-casting, tiny particles are dispersed in a fluid, creating a suspension that is then frozen. Next, the frozen suspension is sublimated — evaporated so that it changes directly from a solid to a gas — and the particles left behind form a microscopically porous mold. The technology can be used in the production in everything from fuel cells to fake meat.
But before the scientists can start thinking about construction projects on Mars, they have to improve freeze-casting on Earth.
“If NASA should decide to use freeze-casting to make something in space, we need to know exactly how this freeze-casting process is going to actually occur,” says Peter Voorhees, the Frank C. Engelhart Professor of Materials Science and Engineering and co-investigator for the ISS project.
Chief among the researchers’ interests is how convection affects the process. On Earth, gravity pulls denser, colder air or fluid toward the ground while less dense, warmer air or fluid rises. When freeze-casting with water, the water’s density decreases as it freezes, causing it to become buoyant and flow upward. The temperature gradient forms currents of warm and cool fluid — convection.
On Earth, where there is gravity, the ice can run into these currents as it forms, disrupting the microstructures of the ice. But what about in space?
“Density differences only matter in gravity,” explains graduate student Krysti Scotti, who is working on the project. And while there is a nominal amount of gravity in space, she said, it isn’t enough to cause noticeable convection. So the scientists are looking to understand how fluid density affects the microstructures.
The plan for the experiments is to use nanoparticles of cupric oxide (CuO) suspended in succinonitrile — a mildly toxic substance that is waxy at room temperature — rather than water or another fluid. Past studies of solidification, which mirrors the steps of freeze-casting up to sublimation, on the ISS have used succinonitrile. That means the Northwestern researchers will have data that they can compare their results to. It’s also a matter of simplicity: the station is already equipped with a furnace that can solidify the substance.
Succinonitrile, like most fluids, has a different set of density features than water: the warmer molecules rise, and the cooler molecules sink. These properties will form the basis of the scientists’ convection studies.
The ISS project enables the most promising freeze-casting experiments in microgravity to date. In 2013, Scotti started testing freeze-casting in microgravity on parabolic flights. These allowed short bursts — about 30 seconds apiece — of spacelike conditions for the experiments.
Since then, Northwestern’s freeze-casting studies have come a long way. A group of undergraduates whom Scotti mentors are in their third year of working toward sending samples into low orbit inside a breadbox-sized device called a “CubeSat,” which is being built by the University of Illinois at Urbana-Champaign. That device will send back data over six months after its 2020 launch, but at its conclusion it will burn up in the Earth’s atmosphere — meaning the researchers won’t see their samples again.
Having astronauts tend to the developing batch of ISS experiments “gives you opportunities to answer questions you can’t answer on the ground,” Voorhees says of the 2021 mission. Plus, they’ll get their samples back for further analysis.
Right now, Voorhees and the other researchers are preparing for a NASA review of their plans later this year. That involves conducting preliminary studies and fine-tuning their proposals.
Scotti and undergraduate Cristabella Wolff are already hard at work in the lab, mixing suspensions of succinonitrile and CuO particles that they will inject into microscope slides they have rigged to induce a temperature gradient. This will give them an up-close view of the suspension, and an opportunity to observe what the particles do after they solidify.
“If the particles are aggregating, that’s going to be a serious problem,” Scotti explains to her fellow researchers during a meeting in Dunand’s office. That’s because the suspension will be in a solid state until it reaches the ISS and gets remelted for the experiments. If the particles are forming masses before freeze-casting, the materials won’t form properly.
Voorhees said he is confident that, barring a shift to a less NASA-friendly U.S. government, the project has a bright future.
“I think we're on a relatively good path towards getting our ticket to fly,” says Voorhees.