Tien Nguyen: This silicon oil bubble is about
to be popped.
As air escapes, the bubble collapses, and
wrinkles form around its edges.
This process is driven by gravity acting on
the thin film—or so scientists thought.
Now, a new study proposes to upend this traditional
understanding of how bubbles in viscous fluids collapse.
By better understanding how bubbles burst,
scientists can control their presence or absence,
which may be useful in a number of industries.
For example, in applications like spray painting,
it’s best to minimize bubbles.
If you’re making ice cream, on the other
hand, it’s better to maximize air bubbles
to make it soft and scoopable, says James
Bird, who led the recent study.
Bird began to question how viscous bubbles
collapse more than a decade ago.
He reasoned that if gravity were the main
force behind a bubble’s collapse, then flipping
it upside down should make the popped film
deflate towards the ground.
Instead what happened is that the upside-down
bubble collapses in the same inward manner
as if it were upright.
So Bird and his coauthors hypothesized that
another force was responsible for viscous
bubble collapse: surface tension.
Yet investigating this theory wasn’t easy.
James Bird: The challenging part is visualizing
it and getting the light just the right way
that you can see wrinkles in what otherwise
would be a transparent film.
Tien Nguyen: The authors estimate that they
watched thousands of silicon bubbles, of varying
viscosities, pop to collect enough data to
measure how fast the bubbles collapsed.
They found that the speed of collapse depended
on viscosity and capillary forces, allowing
them to confirm that surface tension was indeed
the driving force behind the bubbles’ collapse.
The team also investigated why the films wrinkle
as they collapse, which aside from its eye-catching
appearance, has practical implications; these
wrinkles can trap air and cause more bubbles
to form.
Next, Bird says they plan to study how more
complex mixtures of fluids behave so they
can better understand bubbles in the real
world.
