| The properties of water in the vicinity of hydrophobic surfaces are at the origin of many important and poorly understood phenomena. They play an crucial role in the solvation of hydrophobic molecules, attractive hydrophobic forces between macroscopic surfaces, the self-assembly of micelles and lipid membranes, as well as the stability of surface nanobubbles. Moreover, hydrophobic forces are thought to play a decisive role in the folding of proteins and in the transport of water and ions through membranes, i.e. in some of the most central processes in living organisms. Despite the relevance of the subject and the enormous amount of work in the field, many fundamental aspects are still controversial. A simple estimate based on bulk properties shows that liquid water is thermodynamically unstable between two hydrophobic surfaces for separations of less than approximately 1µm. Nevertheless, confined water layers should remain stable down to a distance of about 1nm due to excessive nucleation barriers. This critical distance depends crucially on the presence and on the heavily debated thickness (O(1nm)) of thin gas-like layers that are believed to be present at hydrophobic-water interfaces. Experimentally, much thicker surface nanobubbles complicate the picture and frequently dominate the experimental behavior. The origin and stability of these nanobubbles remain mysterious. Gas-like layers, hydrophobic forces, as well as surface nanobubbles seem to depend on the concentration of dissolved gases and ions, however, conflicting experimental evidence consistently reappears. While numerical studies provide an increasing amount of detailed predictions, many experimental tools in particular for confined water layers rely on indirect effects such as force measurements. In this project, we want to study the stability of liquid confined water layers using a new experimental tool, namely hydrophobic nanochannels etched into glass with a thickness ranging from a few up to a few tens of nanometers. A Fabry-Pérot interferometer incorporated into the nanofluidic chip will allow for measuring the refractive index of the fluid in the channel as a function of both time and position. By varying the temperature and pressure we will force the liquid to evaporate and follow the appearance and growth of gas or vapor bubbles for various conditions (dissolved gas; dissolved ions; pH). To obtain additional information about the dynamics of the transition, we will follow temporal fluctuations of the refractive index (with MHz time resolution) as well as global electric properties of the nanochannels. |
