Researchers in Tokyo use Japan's most powerful computer to explore the role of Ostwald ripening in the formation of bubbles -- as important in engineering as they are in celebratory toasts
Uncork a bottle of champagne, and as the pressure of the liquid is abruptly removed, bubbles immediately form and then rapidly begin the process of "coarsening," in which larger bubbles grow at the expense of smaller ones.
After multi-bubble nucleation, Ostwald ripening occurs because of the "coarsening" interaction between bubbles until only a single bubble remains.
This fundamental nonequilibrium phenomenon is known as "Ostwald ripening," and though it is most familiar for its role in bubbly beverages, it is also seen in a wide range of scientific systems including spin systems, foams and metallic alloys.
On a much larger scale, Ostwald ripening can 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.
So a team of researchers from the University of Tokyo, Kyusyu University and RIKEN in Japan set out to find an answer. In The Journal of Chemical Physics, from AIP Publishing, the researchers describe how they were able to simulate bubble nucleation from the molecular level by harnessing the K computer at RIKEN, the most powerful system in Japan.
At the heart of their work were molecular dynamics simulations. The basic concept behind these simulations is to put some virtual molecules in a box, assign them initial velocities and study how they continue moving -- by using Newton's law of motion to determine their position over time. There were major challenges in doing this, explained Hiroshi Watanabe, a research associate at the University of Tokyo's Institute for Solid State Physics.
"A huge number of molecules, however, are necessary to simulate bubbles -- on the order of 10,000 are required to express a bubble," Watanabe said. "So we needed at least this many to investigate hundreds of millions of molecules -- a feat not possible on a single computer."
The team, in fact, wound up simulating a whopping 700 million particles, following their collective motions through a million time steps -- a feat they accomplished by performing massively parallel simulations using 4,000 processors on the K computer. This was, to the best of their knowledge, the first simulation to investigate multi-bubble nuclei without relying on any artificial conditions.
"In the past, while many researchers wanted to explore bubble nuclei from the molecular level, it was difficult due to a lack of computational power," explained Watanabe. "But now, several petascale computers -- systems capable of reaching performance in excess of one quadrillion point operations per second -- are available around the world, which enable huge simulations."
The team's key finding? The time evolutions of bubbles are well described by a classical theory developed during the 1960s, a mathematical framework called "LSW theory" after its three developers -- Lifshift and Slyozov in Soviet Union and Wagner in Germany. While LSW theory has been shown to hold true for other systems, like ice crystals growing in so-called freezer-burned ice cream, prior to this work nobody had ever shown it also works for describing gas bubbles in liquid.
"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," Watanabe said. "So we were expecting the classical theory to fail to describe the bubble systems, but were surprised to find that it held up."
In other words, although 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.
As far as implications of the team's work, an enhanced understanding of the behavior of bubbles is very important for the field of engineering because it may enable the design of more efficient power stations or propellers.
What's next for the researchers? After exploring cavitation, they're now shifting their focus to boiling. "Bubbles appear when liquid is heated as 'boiling,' or as 'cavitation' when the pressure of the liquid decreases," said Watanabe. "Simulating boiling is more difficult than cavitation at the molecular level, but it will provide us with new knowledge that can be directly applied to designing more efficient dynamo."
The team is also targeting a polymer solution. "Surfactants make bubbles stable, while defoamers make them unstable," he added. "Recent developments in computational power will allow us to simulate these kinds of complex systems at the molecular level."
The article, "Ostwald ripening in multiple-bubble nuclei," is authored by Hiroshi Watanabe, Masaru Suzuki, Hajime Inaoka and Nobuyasu Ito. It will appear in The Journal of Chemical Physics on December 18, 2014 (DOI: 10.1063/1.4903811). After that date, it can be accessed at: http://scitation.aip.org/content/aip/journal/jcp/141/23/10.1063/1.4903811
The authors of this paper are affiliated with the University of Tokyo, Kyusyu University and the Japan Advanced Institute for Computational Science.
After multi-bubble nucleation, Ostwald ripening occurs because of the "coarsening" interaction between bubbles until only a single bubble remains. CREDIT: H.Inaoka/RIKEN
ABOUT THE JOURNAL
The Journal of Chemical Physics publishes concise and definitive reports of significant research in the methods and applications of chemical physics. See: http://jcp.aip.org
Jason Socrates Bardi, AIP | newswise
Ultra-compact phase modulators based on graphene plasmons
27.06.2017 | ICFO-The Institute of Photonic Sciences
Smooth propagation of spin waves using gold
26.06.2017 | Toyohashi University of Technology
An international team of scientists has proposed a new multi-disciplinary approach in which an array of new technologies will allow us to map biodiversity and the risks that wildlife is facing at the scale of whole landscapes. The findings are published in Nature Ecology and Evolution. This international research is led by the Kunming Institute of Zoology from China, University of East Anglia, University of Leicester and the Leibniz Institute for Zoo and Wildlife Research.
Using a combination of satellite and ground data, the team proposes that it is now possible to map biodiversity with an accuracy that has not been previously...
Heatwaves in the Arctic, longer periods of vegetation in Europe, severe floods in West Africa – starting in 2021, scientists want to explore the emissions of the greenhouse gas methane with the German-French satellite MERLIN. This is made possible by a new robust laser system of the Fraunhofer Institute for Laser Technology ILT in Aachen, which achieves unprecedented measurement accuracy.
Methane is primarily the result of the decomposition of organic matter. The gas has a 25 times greater warming potential than carbon dioxide, but is not as...
Hydrogen is regarded as the energy source of the future: It is produced with solar power and can be used to generate heat and electricity in fuel cells. Empa researchers have now succeeded in decoding the movement of hydrogen ions in crystals – a key step towards more efficient energy conversion in the hydrogen industry of tomorrow.
As charge carriers, electrons and ions play the leading role in electrochemical energy storage devices and converters such as batteries and fuel cells. Proton...
Scientists from the Excellence Cluster Universe at the Ludwig-Maximilians-Universität Munich have establised "Cosmowebportal", a unique data centre for cosmological simulations located at the Leibniz Supercomputing Centre (LRZ) of the Bavarian Academy of Sciences. The complete results of a series of large hydrodynamical cosmological simulations are available, with data volumes typically exceeding several hundred terabytes. Scientists worldwide can interactively explore these complex simulations via a web interface and directly access the results.
With current telescopes, scientists can observe our Universe’s galaxies and galaxy clusters and their distribution along an invisible cosmic web. From the...
Temperature measurements possible even on the smallest scale / Molecular ruby for use in material sciences, biology, and medicine
Chemists at Johannes Gutenberg University Mainz (JGU) in cooperation with researchers of the German Federal Institute for Materials Research and Testing (BAM)...
19.06.2017 | Event News
13.06.2017 | Event News
13.06.2017 | Event News
27.06.2017 | Physics and Astronomy
27.06.2017 | Life Sciences
27.06.2017 | Earth Sciences