New finding on bubbles challenges old view
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Bubbles do not form by the same mechanism as in the case of condensing droplets
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The molecular mechanism for bubble formation and phase transition is not yet understood
The insights into phase transition could translate into practical and safety benefits for industry
New findings by chemical engineers have cast doubt on the conventional theory of bubble formation in liquids, dating back to the 1920s. In the conventional view, a liquid boiling and turning into a vapour takes place in a systematic process known as ‘nucleation and growth.’
Microscopic bubbles
The liquid first forms tiny ‘nuclei,’ or microscopic bubbles, that eventually grow as they pick up particles like a snowball gaining size as it rolls down a hillside. This conventional view is described by the ‘classical nucleation theory,’ which was originally proposed in the 1920s.
“The findings cast into doubt some aspects of the theory that attempts to describe the underlying molecular mechanism behind a phenomenon called homogeneous nucleation,” said David S. Corti, an associate professor of chemical engineering at Purdue University who was involved in the research.
As water is heated in a pot on a stovetop, it begins boiling when the temperature reaches 100 degrees Celsius. “You get little microscopic bubbles that form on the surfaces of the pot,” Corti said.
This bubble formation on a surface is called heterogeneous nucleation. Bubbles also may form, however, by homogeneous nucleation, in which they appear not on surfaces, but within the liquid itself. The new findings specifically apply to homogeneous nucleation, according to a Purdue University press release. The conventional nucleation theory uses the same mechanism for how liquid droplets condense from a vapour in attempts to describe how bubbles form in a liquid.
Saturation pressure
The Purdue researchers found, however, that bubbles do not form by the same mechanism as in condensing droplets, Corti said.
Nucleation occurs when a liquid is heated above its boiling temperature or when the pressure exerted on a liquid is decreased below the so-called saturation pressure, which is the case when the lid is removed from the bottle of a carbonated beverage. “A common example is when you heat water in a microwave oven,” Corti said. “It heats liquid from the inside as opposed to on the surface, so you can actually raise the temperature of the water above 100 degrees Celsius and it doesn’t boil. Sometimes when you microwave water in a mug you can superheat it and, if you put a spoon in there after removing it from the microwave, you introduce nucleation sites and it boils off and sprays hot water. The transition happens rapidly, causing a vapour explosion.”
According to the conventional theory, the pathway going from a liquid to a vapour is narrow, strictly defining the molecular mechanism by which the liquid becomes a vapour.
“You could think of this pathway as a mountain pass,” Corti said. “In order to get from the liquid to the vapour, you have to go over this mountain pass. If you climb up and you’re not quite at the top, sometimes you can roll back down, but if you get to the top, you can roll down to the other side and get to the vapour phase.”
The new research has shown that this metaphorical mountain pass is actually more broad and flat than previously thought, meaning there are several possible pathways responsible for the phase transition.
At the same time, what the researchers found was that once you get over this mountain pass, which is called the free energy surface of bubble formation, the mountain disappears and the bubbles have no choice but to plummet into the vapour phase.
The findings are detailed in a research paper in the journal Physical Review Letters. Apart from examples from daily life, nucleation also occurs in the chemical industry and in other work environments where liquids flow through pip
es, sometimes with undesirable consequences.
“Depending on the diameter of the pipes, the pressure of the liquid can drop very rapidly, causing it to become superheated, and before the pressure recovers you can get this phase transition,” Corti said.
The bubbles that form can then collapse when the pressure increases again, sometimes causing significant damage to equipment. New insights into phase transition could translate into practical and safety benefits for industry.
Sonochemistry
Such insights also could result in a better understanding of the mechanisms responsible for ‘sonochemistry’ and ‘sono-luminescence’ processes in which sound waves are used to form tiny bubbles in liquids. As the bubbles collapse, they emit flashes of light and generate high pressures and temperatures that could be used to enhance chemical reactions.
Although the new findings indicate current theory does not adequately describe the molecular mechanism for bubble formation and phase transition from a liquid to vapour, the Purdue researchers do not yet know precisely what that mechanism is. — Our Bureau
The molecular mechanism for bubble formation and phase transition is not yet understood
The insights into phase transition could translate into practical and safety benefits for industry
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