In the conventional view, a liquid boiling and turning into a vapor
takes place in a systematic process known as "nucleation and growth."
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 "classical
nucleation theory," which was originally proposed in the 1920s.
"Our findings indicate that this is not what's going on," Corti said.
"The bubble grows via a mechanism very different from classical
nucleation theory."
Findings are detailed in a research paper appeared
online in the journal Physical Review Letters. The paper was written
by Corti and chemical engineering doctoral student Mark Uline.
As water is heated in a pot on a stovetop, it begins boiling when the
temperature reaches 100 degrees Celsius, or 212 degrees Fahrenheit.
"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.
"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 vapor explosion."
The conventional nucleation theory uses the same mechanism for how
liquid droplets condense from a vapor in attempts to describe how
bubbles form in a liquid. The Purdue researchers found, however, that
bubbles do not form by the same mechanism as condensing droplets,
Corti said.
According to the conventional theory, the pathway going from a liquid
to a vapor is narrow, strictly defining the molecular mechanism by
which the liquid becomes a vapor.
"You could think of this pathway as a mountain pass," Corti said. "In
order to get from the liquid to the vapor, 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 vapor 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 we found is that once you get over this
mountain pass, which is called the free energy surface of bubble
formation, the surface disappears," Corti said. "You look at one side
and you see the mountain and think everything is OK, but once you
climb over, it's as if the mountain disappears on the other side."
In the conventional view, the forming bubbles moving down the mountain
pass could, in principle, reverse direction back toward the liquid
phase.
"But in our view, as soon as you get over the top of the mountain, the
mountain disappears," Corti said. "You have no choice but to plummet
to something else, the vapor phase."
The findings were based on research using new theoretical methods and
verified by computational simulations developed by the Purdue
engineers.
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 such as
champagne, beer or a soft drink.
"This also occurs in the chemical industry and in other work
environments where liquids flow through pipes, sometimes with
undesirable consequences," Corti said. "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."
The bubbles that form can then collapse when the pressure increases
again, sometimes causing significant damage to equipment.
In other industrial processes involving propeller blades, bubbles can
form or undergo "cavitation" and then suddenly implode, producing high
temperatures and extreme pressures and damaging equipment.
"There are tons of examples, but the real fundamental mechanism
underlying what's going on is not that well understood, even for very
simple systems," Corti said.
New insights into phase transition could translate into practical and
safety benefits for industry. Such insights also could result in a
better understanding of the mechanisms responsible for initiating "sonochemistry"
and "sonoluminescence" 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.
Another potential practical benefit is to improve the manufacture of
foams made of plastic polymers that depends on the formation and
distribution of bubbles.
Although the new findings indicate current theory does not adequately
describe the molecular mechanism for bubble formation and phase
transition from a liquid to vapor, the Purdue researchers do not yet
know precisely what that mechanism is.
"We are still working out the full implications of this ourselves,"
Corti said.
|