By Hamish Johnston
Bubbles are wonderful things – as well as giving children hours of fun, they provide physicists with a number of fascinating phenomena to study and genuine mysteries to solve.
One curious effect that physicists have known about for some time is that tiny air bubbles in water will last much longer when they are stuck on a surface – rather than floating freely. A free bubble with a diameter of 100 nm or less will only survive a few microseconds, while a bubble of similar size on a surface can endure for days.
Why is this interesting, you might wonder? For one thing, controlling nanobubbles can be very important when designing tiny machines that shift fluids about. A coating of nanobubbles could make it easier for a fluid to flow along a tiny channel. Conversely, bubbles in the wrong place could gum up the works. Nanobubbles could someday be designed to carry drugs to specific places in the body, popping on arrival.
Such applications could be one step closer thanks to work published today in Physical Review Letters. Joost Weijs and Detlef Lohse at the MESA+ Institute for Nanotechnology at the University of Twente in the Netherlands have devised a theoretical model that tries to explain why bubbles on a surface stick around for so much longer.
The pair say that two important effects are at play. The first – and most obvious – is that bubbles stuck to a surface aren’t spherical, but rather are flattened out on the surface. This means that they have radii of curvature that are much larger than spheres of similar volume. It’s well known that the smaller the radius of curvature, the faster that gas leaks from a bubble.
The second reason is related to the fact that the nanobubbles are fixed on a surface and tend to be surrounded by other nanobubbles. This means that the liquid in the vicinity of the surface becomes saturated with escaping gas molecules that must diffuse away. This puts the brakes on gas that is trying to diffuse out of the bubbles. Free bubbles don’t have this problem because as they rise in a liquid, they move away from gas molecules that they have released.
You can read the paper here.
This isn’t the first time that a paper has appeared in Physical Review Letters about nanobubbles. In 2011 Lohse and two other colleagues at Twente published this novel proposal about how nanobubbles on surfaces could be recycling gas molecules.