Findings
show how to make confined bubbles develop uniformly, instead of in their usual
scattershot way.
David L. Chandler | MIT News Office
David L. Chandler | MIT News Office
June 17, 2019 -- The
formation of air bubbles in a liquid appears very similar to its inverse
process, the formation of liquid droplets from, say, a dripping water faucet.
But the physics involved is actually quite different, and while those water
droplets are uniform in their size and spacing, bubble formation is typically a
much more random process.
Now,
a study by researchers at MIT and Princeton
University
The
new findings could have implications for the development of microfluidic
devices for biomedical research and for understanding the way natural gas
interacts with petroleum in the tiny pore spaces of underground rock
formations, the researchers say. The findings are published today in the journal
PNAS, in a paper by MIT
graduate Amir Pahlavan PhD ’18, Professor Howard Stone of Princeton, MIT School
of Engineering Professor of Teaching Innovation Gareth McKinley, and MIT
Professor Ruben Juanes.
The
key to producing uniformly sized and spaced bubbles lies in confining them to a
narrow space, Juanes explains. When air or gas is released into a large
container of liquid, the dispersal of bubbles is scattershot. When released
into liquid that is confined in a relatively narrow tube, however, the gas will
produce a stream of bubbles perfectly matched in size, and forming at even
intervals. This uniform and predictable behavior, independent of specific
starting conditions, is known as universality.
The
process of formation of droplets or bubbles is very similar, beginning with an
elongation of the flowing material (whether it’s air or water), and eventually
a thinning and pinch-off of the “neck” connecting the droplet or bubble to the
flowing material. That pinch-off then allows the droplet or bubble to collapse
into a spherical shape. Picture blowing soap bubbles: As you blow through the
ring, a tube of soap film gradually extends outward in a long pouch before
pinching off to form a round bubble that floats away.
“The
process of a droplet dripping from a faucet is known to be universal,” says
Juanes, who has a joint appointment in the departments of Civil and
Environmental Engineering and Earth, Atmospheric and Planetary Sciences. If the
dripping liquid has a different viscosity or surface tension, or if the opening
of the faucet is a different size, “it doesn’t matter. You can find
relationships that allow you to determine a master curve or a master behavior
for describing that process,” he says.
But
when it comes to what is, in a sense, the opposite process to a dripping faucet
— the injection of air through an opening into a large tank of liquid such as a
Jacuzzi tub — the process is not universal. “So if you have irregularities in
the orifice, or if the orifice is larger or smaller, or if you inject with some
pulsation, all of that will lead to a different pinch-off of the bubbles,”
Juanes says.
The
new experiments involved gas percolating onto viscous liquids such as oil. In
an unconfined space, the sizes of the bubbles are unpredictable, but the situation
changes when they bubble into liquid in a tube instead. Up to a certain point,
the size and shape of the tube doesn’t matter, nor do the characteristics of
the orifice the gas comes through. Instead the bubbles, like the droplets from
a faucet, are uniformly sized and spaced.
Pahlavan
says, “Our work is really a tale of two surprising observations; the first
surprising observation came around 15 years ago, when another group
investigating formation of bubbles in large liquid tanks observed that the pinch-off
process is nonuniversal” and depends on the details of the experimental setup.
“The second surprise now comes in our work, which shows that confining the
bubble inside a capillary tube makes the pinch-off insensitive to the details
of the experiment and therefore universal.”
This
observation is “surprising,” he says, because intuitively it might seem that
bubbles able to move freely through the liquid would be less affected by their
initial conditions than those that are hemmed in. But the opposite turned out
to be true. It turns out that interactions between the tube and the forming
bubble, as a line of contact between the air and the liquid advances along the
inside of the tube, play an important role. This “effectively erases the memory
of the system, of the details of the initial conditions, and therefore restores
the universality to the pinch-off of a bubble,” he says.
While
such research may seem esoteric, its findings have potential applications in a
variety of practical settings, Pahlavan says. “Controlled generation of drops
and bubbles is very desirable in microfluidics, with many applications in mind.
A few examples are inkjet printing, medical imaging, and making particulate
materials.”
The
new understanding is also important for some natural processes. “In geophysical
applications, we often see fluid flows in very tight and confined spaces,” he
says. These interactions between the fluids and the surrounding grains are
often neglected in analyzing such processes. But the behavior of such geological
systems is often determined by processes at the grain-scale, which means that
the kind of microscale analysis done in this work could be helpful in
understanding even such very large-scale situations.
The
bubble formation in such geological formations can be a blessing or a curse,
depending on the context, Juanes says, but either way it’s important to
understand. For carbon sequestration, for example, the hope is to pump carbon
dioxide, separated out from power plant emissions, into deep formations to
prevent the gas from getting out into the atmosphere. In this case, the
formation of bubbles in tiny pore spaces in the rock is an advantage, because
the bubbles tend to block the flow and keep the gas anchored in position,
preventing it from leaking back out.
But
for the same reason, bubble formation in a natural gas well can be a problem,
because it can also block the flow, inhibiting the ability to extract the
desired natural gas. “It can be immobilized in the pore space,” he says. “It
would take a much greater pressure to be able to move that bubble.”
“This
is a very nice and careful piece of work,” says Jens Eggers, a professor of
applied mathematics at the University
of Bristol , in the U.K.
These
findings, he says, reflect the fact that “there is a lot more complexity
to
problems like pinch-off than previously thought.” Eggers adds that “Of course,
understanding this complexity is crucial for applications, where one does not
have a choice to pick a particularly simple part of the problem, but has to
face all the complications.”
http://news.mit.edu/2019/how-gas-bubbles-form-liquid-0617
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