Sticky and Slippery Nanobubbles

The presence of bubbles only twenty to thirty nanometers tall appears to explain why some surfaces can be surprisingly slick or unusually sticky. The bubbles form on the surface of hydrophobic (water repellent) materials immersed in water. The stickiness arises as two hydrophobic surfaces approach one another and the bubbles link them together, leading to an attractive force with a range related to nanobubble height. Alternatively, the bubbles can serve as a kind of lubricant by forming a layer that allows water to slip smoothly over certain materials - such as the hydrophobic fabric of Olympic swimming suits.

Until recently, however, evidence of nanobubbles has been largely circumstantial because they are so difficult to detect. The bubbles are too small to image with light, and too fragile to probe with most contact techniques that use tiny mechanical probes to measure molecular scale features.

A group at the Ian Wark Research Institute of the University of South Australia (Phil Attard, phil.attard@unisa.edu.au, 618-8302-3564) has now obtained the first direct images of nanobubbles on hydrophobic surfaces. To acquire the images, the researchers gently examined glass surfaces with a tapping-mode atomic force microscope (AFM), which consisted of a conventional AFM probe tip attached to a vibrating cantilever that scanned across samples immersed in water. The groundbreaking images revealed that nanobubbles form closely packed, irregular networks that cover hydrophobic surfaces nearly completely, and that the bubbles rapidly reform after they are disturbed.

The work also seems to have solved a mystery regarding how the miniscule bubbles can exist at all. Pressure inside a bubble is related to the curvature of the bubble's surface - the smaller a spherical bubble is, the higher both the curvature and the pressure must be. High pressures, however, would cause the trapped gasses to rapidly dissolve into the surrounding water, and the bubbles should spontaneously disappear. The tapping-mode AFM resolves this paradox by showing that the nanobubbles are not round, but flattened like pancakes, with curvature and pressure much lower than previously expected. (J. W. G. Tyrrell and P. Attard, Physical Review Letters, 22 October 2001.)