Surface nanobubbles or Knudsen bubbles?

The mysterious stability of nanobubbles on surfaces is a puzzle baffling soft matter and colloid scientists. Bubbles inside a fluid tend to be spherical, but surface bubbles have the appearance of blisters with typical widths of 1000 nanometers (nm) and heights of 20 nm. The existence of surface bubbles was proposed to explain the extremely long range and the magnitude of the strongly attractive forces observed between hydrophobic surfaces in water [1]. Nanobubbles are of interest because they are easily produced and are stable, and as such, their presence may be altering many aqueous interfaces and exerting influence on processes as diverse as froth flotation to the transportation of anticancer drugs across membranes. Classically, bubbles will deflate, leading to an increase in Laplace pressure (the pressure differential inside and outside a bubble) and a positive feedback loop that results in their rapid disappearance. However, surface nanobubbles, seemingly unaware of the rules, can remain stable for days. Now, writing in Physical Review Letters, James Seddon and coauthors [2] at the University of Twente, the Netherlands, have proposed an explanation for this stability, whereby the properties of the gas within a nanobubble generate a recirculation of the surrounding liquid, which effectively ensures that the gas escaping the bubble through diffusion is recaptured and the bubble lifetime is extended.