Simulating the power of bubbles

Researchers have used the most powerful computer in Japan to explore a process observed in both bubbly beverages as well as scientific systems including spin systems, foams and metallic alloys. As a bottle of champagne is uncorked, the pressure of the liquid is abruptly removed, bubbles immediately form and they then rapidly begin the process of ‘coarsening’, in which larger bubbles grow at the expense of smaller ones.

This fundamental non-equilibrium phenomenon, known as ‘Ostwald ripening’, can also be observed in a power-generating turbine. Most power stations rely on boilers to convert water into steam, but the phase transition involved is highly complex. During the phase transition, no-one is exactly sure what’s occurring inside the boiler - especially how bubbles form.

Researchers from Japan’s University of Tokyo, Kyushu University and RIKEN set out to simulate bubble nucleation from the molecular level by harnessing RIKEN’s powerful K computer. Through molecular dynamics simulations, the researchers would put some virtual molecules in a box, assign them initial velocities and study how they continue moving, using Newton’s law of motion to determine their position over time.

Hiroshi Watanabe, from the University of Tokyo’s Institute for Solid State Physics, explained the challenges of the study, with at least 10,000 molecules required to express a single bubble. “So we needed at least this many to investigate hundreds of millions of molecules - a feat not possible on a single computer,” he said.

The team wound up simulating 700 million particles, following their collective motions through a million time steps, which they accomplished by performing parallel simulations using 4000 processors on the K computer. Their results have been published in the Journal of Chemical Physics.

The time evolutions of bubbles are well described by the mathematical framework ‘LSW theory’, developed during the 1960s, but until now, nobody had ever shown it also works for describing gas bubbles in liquid. Watanabe said, “While the nucleation rate of droplets in condensation is well predicted by the classical theory, the nucleation rates of bubbles in a superheated liquid predicted by the theory are markedly different from the values observed in experiments. So we were expecting the classical theory to fail to describe the bubble systems, but were surprised to find that it held up.”

So while Watanabe and colleagues had hoped their simulation would provide clues to help clarify why the classical theory fails to predict the rate of bubble nucleation, it remains a mystery. However, their work to enhance understanding of the behaviour of bubbles may enable the design of more efficient power stations or propellers.

The team will now shift their focus to boiling, which Watanabe says is “more difficult than cavitation at the molecular level, but … will provide us with new knowledge that can be directly applied to designing more efficient dynamo”. The researchers are also targeting a polymer solution.

“Surfactants make bubbles stable, while defoamers make them unstable,” Watanabe said. “Recent developments in computational power will allow us to simulate these kinds of complex systems at the molecular level.”

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