Physics and Engineering

Team Is First to Model Phase Change in Fluids

Processes as diverse as decaffeinating coffee and improving fuel cell efficiency in cars of the future could be affected by a new finding by two Rochester professors.

Yonathan Shapir, a professor of physics and chemical engineering, and Eldred Chimowitz, a professor in the Department of Chemical Engineering, have come up with a mathematical model that will allow scientists to simulate and understand the increasingly complex way molecules behave during phase changes, the transformation process when matter moves from one phase to another, such as from liquid to gas.

“This problem has baffled scientists for decades,” says Shapir. “This is the first time a computer program could simulate a phase transition, because the computers would always bog down at what’s known as the ‘critical slowdown.’ We figured out a way to perform a kind of end-run around that critical point slowdown, and the results allow us to calculate certain critical point properties for the first time.”

“Critical slowdown” is a phenomenon that happens as matter nears the point where it moves from one phase to another. For instance, as molecules in a gas are cooled, they lose some of their motion, but are still moving around and bumping into each other. As the temperature drops to where the gas will change into a liquid, the molecules’ motion becomes correlated across larger and larger distances.

That correlation is a bit like deciding where to go to dinner—quick and easy with two people but takes forever for a group of 20 to take action. The broadening correlation dramatically increases the time it takes for the gas to reach an overall equilibrium, and that directly leads to an increase in computing time.

In findings published in Physical Review Letters, the research team of Shapir, Chimowitz, and physics graduate student Subhranil De reported that they have devised a computational model consisting of two separate reservoirs of fluid at equilibrium and near the critical point threshold. One reservoir was slightly more pressurized than its neighbor. The reservoirs were opened to each other and the pressure difference caused the fluids to mix.

The team let the simulation run until the entire system reached thermodynamic equilibrium. By watching the rate that equilibrium returned, the team was able to calculate the behavior at the critical point.

The researchers’ findings match predictions and experimental results, including very precise measurements performed in microgravity on the Space Shuttle.

—Jonathan Sherwood