AFM Characterizes Bubbles that Should Not Be

David L. Shenkenberg

Nanobubbles should not exist because they defy a law of physics. Bubbles with nanometer-size radii should have high pressure within them that causes them to be squeezed out in less than a second, according to LaPlace’s Law. However, nanobubbles last for hours.

First discovered with atomic force microscopy (AFM), nanobubbles were thought possibly to be an artifact of the technique. Later, cryoelectron microscopy confirmed their existence. Now researchers from the University of Twente in Enschede, the Netherlands, have used AFM to discover new properties of nanobubbles.

Researchers used atomic force microscopy to study the properties of nanometer-size bubbles that, according to LaPlace’s Law, should not exist. The bubble is the light-colored spot in this atomic force microscopy image.

Understanding how these bubbles form could provide clues for building micro- and nanofluidic devices, said principal investigator Detlef Lohse. Because bubbles reduce friction, more bubbles will increase the speed of fluid flow.

The scientists observed the nanobubbles in pure water on a silicon wafer that was coated with a silane derivative to make the surface hydrophobic, increasing the probability of generating nanobubbles. To image the bubbles, they employed an atomic force microscope from Molecular Imaging Corp. of Tempe, Ariz. They used the instrument in tapping mode because the setting was not likely to pop the bubbles, whereas contact mode would. They employed an atomic force microscope tip from MikroMasch SL of Madrid, Spain, the ultrasharp point of which enabled them to resolve the bubbles.

The investigators observed a nanobubble traveling along a groove, and they believe that the groove and the AFM tip influenced the direction of the motion. By measuring the surface of the wafer with and without bubbles, they found that the bubbles formed in energetically favorable places and that indentions and ridges of the surface were much smaller than the bubbles.

It was determined that immersing the wafer in heated water greatly increased the number of nanobubbles, and the bubbles did not disappear when the water cooled to room temperature. From these results, the researchers concluded that the conditions during immersion are decisive. Next they pumped CO₂ gas into the immersed wafer and saw that the density of the bubbles increased proportionally with the amount of gas.

They discovered that cleaning the wafer with ethanol before adding water greatly increased the number of bubbles, for as-yet-unknown reasons. The scientists ascertained that adding a surfactant (butanol) decreased the number of bubbles. Finally, they reproduced an experiment that showed that the bubbles can be created by exchanging ethanol and water.

The investigators found that the surface markedly affects nanobubbles, and they are now exploring ultrasmooth surfaces, Lohse said. They will work with carbon because it has a well-defined crystalline structure and because, unlike silane, it is a pure element.

Langmuir, June 19, 2007, pp. 7072-7077.