Bubbles at the boundary

New measurements may settle a long controversy about what happens when water comes into contact with a hydrophobic surface

Credit: © 2007 APS

'Like oil and water' is a catch-all phrase for two things that don't mix, but, surprisingly, such hydrophobic interactions are still not fully understood from a microscopic perspective. For example, some researchers have proposed that the unusual behaviour seen at the interface between water and hydrophobic surfaces such as polymers can be explained by the presence of 'nanobubbles' at the boundary. However, the results of experiments have been contradictory.

Now, William Ducker and colleagues¹ at the University of Melbourne in Australia have found further evidence for the existence of nanobubbles. Starting with a silicon surface that has been treated to make it hydrophobic, the group first expose the surface to carbon dioxide gas dissolved in ethanol, and then to water. The ethanol step is important as it ensures that the water is super-saturated with the gaseous bubbles. The shape of the bubbles and the density of the CO₂ gas are determined from atomic force microscopy and optical absorption measurements, respectively. In addition to finding new evidence for nanobubbles, Ducker and co-workers show that the gentle curvature of the bubbles allows them to remain stable at the interface over long periods of time.

The nanobubble hypothesis has been developed to explain unexpected lubrication that occurs near hydrophobic interfaces. These findings may therefore prove useful for increasing fluid flow in narrow channels.