Materials Science and Technology of Polymers

Super-stable and super-mysterious Nanobubbles

Super-stable and super-mysterious
Nanobubbles - extremely small air bubbles that reside on surfaces at the solid-liquid interface - are not only unexpectedly stable, but they are super-stable. This is the conclusion of a study carried out jointly in the groups of Prof. Detlef Lohse and Prof. G. Julius Vancso which was published this week in Physical Review Letters.
The experiments show that the mystery of these air bubbles is even larger than expected before.
In a second publication the groups of Prof. Detlef Lohse and Prof. Harold Zandvliet report further on the detailed characterization of the behavior of these nanobubbles. This study was published also this week in Langmuir.
A material that is submersed in water may show at its surface nanobubbles. Since these air bubbles have dimensions much smaller than the wavelength of visible light, the researchers applied an Atomic Force Microscope (AFM) to detect the bubbles. The AFM tip can indeed visualize the bubbles as elevated round features on the stiff substrates utilized. Due to the small dimensions of the bubbles (100 nm diameter) it is theoretically expected that the surface tension presses the gas within seconds out of the bubble. However, what has been observed? The bubbles are stable for periods over many hours. Can we blow them up by reducing the pressure inside the liquid, was one of the questions posed by the researchers. This can be realized by a process called cavitation in which a shock wave is used to cause the explosion of the bubbles to larger, optically visible bubbles. But to a big surprise, the expected outcome was not observed. Graduate student Bram Borkent (member of the Physics of Fluids group of Prof. Detlef Lohse) said: " There must be something really interesting at hand, something we do not quite understand (yet)". Following the cavitation experiment the researchers investigated the sample surface again by AFM, and observed that the entire surface was densely covered with nanobubbles. Thus the bubbles survived the shock wave without cavitating. "Super-stable" one could conclude. In the Langmuir paper graduate student Shangjiong Yang showed how sensitive the nanobubbles react to temperature, gas concentration and the manner of preparing the surfaces. "The surface seems to have some kind of memory since the density of bubbles depends on the type of alcohol used for cleaning the surfaces" says Yang. In the staedily prgressing miniaturization, e.g. in the field of micro- and nanofluidics (manipulation of liquids in channels with micrmeter or nanometer size dimensions), nanobubbles may play an important practical role, e.g. to manipulate the friction experienced by the liquid inside the channels.

Image right
AFM 3D-visualization of nanobubbles on a hydrophobized silicon wafer in water. The height of the nanobubbles is < 20 nm.

Note for the press
The article "Superstability of surface nanobubbles" of Bram Borkent, Stephan Dammer, Holger Schonherr, Julius Vancso, en Detlef Lohse was published in Physical Review Letters. A pdf-file can be sent upon request.

The article "Characterization of nanobubbles on hydrophobic surfaces in water" of Shangjiong Yang, Stephan Dammer, Nicolas Bremond, Harold Zandvliet, Stefan Kooij, en Detlef Lohse appears in Langmuir. A pdf-file can be sent upon request.
Contact person for the press: Wiebe van der Veen, (053) 4894244