Title：Surface nanobubbles and nanodroplets: the big picture

Reporter：Prof. Detlef Lohse

Time：14:00-16:00pm, Apr. 12, 2017（ Wednesday）

Place：Lecture Hall, Department of Thermal Engineering

Abstract：

Surface nanobubbles are nanoscopic gaseous domains on immersed substrates which can survive for days. They were first speculated to exist about 20 years ago, based on stepwise features in force curves between two hydrophobic surfaces, eventually leading to the first atomic force microscopy (AFM) image in 2000. While in the early years it was suspected that they may be an artefact caused by AFM, meanwhile their existence has been confirmed with various other methods, including through direct optical observation. Their existence seems to be paradoxical, as a simple classical estimate suggests that they should dissolve in microseconds, due to the large Laplace pressure inside these nanoscopic spherical-cap-shaped objects. Moreover, their contact angle (on the gas side) is much smaller than one would expect from macroscopic counterparts. This review will not only give an overview on surface nanobubbles, but also on surface nanodroplets, which are nanoscopic droplets (e.g. of oil) on (hydrophobic) substrates immersed in water, as they show very similar properties and can easily be confused with surface nanobubbles and as they are produced in a very similar way, namely by a solvent exchange process, leading to local oversaturation of the water with gas or oil, respectively, and thus to nucleation.

We will briefly report how surface nanobubbles and nanodroplets can be made, how they can be observed (both individually and collectively), and what their properties are. We will then explain the long lifetime of the surface nanobubbles. The crucial element is pinning of the three-phase contact line at chemical or geometric surface heterogeneities. The dynamical evolution of the surface nanobubbles then follows from the diffusion equation, Laplace’s equation, and Henry’s law. In particular, one obtains stable surface nanobubbles when the gas influx from the gas-oversaturated water and the outflux due to Laplace pressure balance. This is only possible for small enough surface bubbles. It is therefore the gas oversaturation ζ which determines the contact angle of the surface nanobubble or nanodroplet and not the Young equation. The talk reports on joint work with Xuehua Zhang.

Brief Biography：

Lohse graduated from the University of Bonn in 1989 with a degree in Physics, and completed his PhD at the University of Marburg in 1992. He served as a postdoctoral research fellow at the University of Chicago from 1993 to 1995.

Detlef Lohse was appointed as Chair at the University of Twente in 1998 (aged 34) and built up the Physics of Fluids group. Nearly 30 former group members meanwhile hold tenured professorships elsewhere. Lohse’s present research interests include turbulence and multiphase flow, granular matter, and micro- and nanofluidics.

Lohse’s two most important scientific achievements are his contributions to the explanation of single-bubble sonoluminescence and to a better understanding of thermally driven turbulence. For these achievements he received the Spinoza Prize in 2005, the most prestigious award in science in the Netherlands. In 2009 he has received the Simon-Stevin Master Prize for his work on inkjet printing. He is the recipient of the inaugural, and the most prestigious fluid mechanics prize, the Batchelor Prize, in 2012, for best research in fluid mechanics in the last ten years. He is a member of the German Academy of Sciences, and a member of the Royal Dutch Academy of Sciences. He has published about 350 publications in refereed scientific journals, including 9 Nature/Science papers, 63 Physical Review Letters, 2 Review of Modern Physics, 1 Annual Reviews of Fluid Mechanics.