Phys. Rev. Lett. 109, 066102 (2012) http://arxiv.org/abs/1204.2633 (2012)

Nanoscale bubbles formed at the surface of an immersed solid can be surprisingly stable, surviving for days rather than the microseconds predicted by classical diffusion, and may play a role in a variety of interfacial processes. Atomic force microscopy has previously been used to image these nanobubbles, but the technique is intrusive and relatively slow, which has left important questions about their formation and dynamics unanswered. Two independent research teams have now shown that optical microscopy can image surface nanobubbles and probe their growth dynamics on a timescale of seconds.

Stefan Karpitschka and colleagues at the Max Planck Institute of Colloids and Interfaces, and the University of Twente used optical interference-enhanced reflection microscopy to visualize nanobubbles formed on a silicon surface covered with a hydrophobic monolayer. Their approach relies on the fact that the intensity of locally reflected light is different for areas of surface with and without nanobubbles, and uses a layered surface to increase the contrast. The bubbles are formed using a technique in which the water covering the surface is replaced with ethanol, which is then in turn replaced with water saturated with gas. The interference-enhanced imaging showed that the nanobubbles are formed a few seconds after the ethanol is replaced with water and that the bubbles are stable up to 65 °C.

Alternatively, Chon Chan and Claus-Dieter Ohl of Nanyang Technological University used total internal reflection fluorescence microscopy to visualize nanobubbles on a hydrophilic glass surface. The bubbles, which are also formed using water–ethanol exchange, are imaged through the glass in a thin volume of liquid using a fluorescent dye to enhance the contrast.