Interfacial phenomena in micro- and nanofluidics: nanobubbles, cavitation, and wetting

In this thesis, we have studied water in contact with a hydrophobic surface. The thesis covers three interfacial phenomena which can occur in such a system: part I - spherically cap-shaped gas bubbles (“surface nanobubbles”) residing on atomically smooth surfaces (10-100 nm). II - gas pockets, trapped in extremely small surface defects, growing to micrometer sized vapor bubbles (”cavitation”) (100-1000 nm); III - wetting dynamics of a rough, superhydrophobic surface (1-10µm). Our main result in part I is that the observed nanobubble shape does not depend on intrinsic cantilever properties, used in detecting the bubbles. Furthermore, we find that the nanoscopic contact angle (measured through the water) does not depend on the nanobubble radius and is much smaller (120 deg) than has hitherto been reported (~ 160 deg). Contamination is the most likely candidate to explain the latter observation. In part II, our main result is extremely little gas pockets (100nm) serve as nucleation sites when the liquid pressure is lowered sufficiently and cavitation bubbles occur. The minimum pressure which is needed to nucleate the bubbles is inversely proportional to the pit radius and is in excellent agreement with the crevice model theory as developed in 1989. Hence, the origin of cavitation inception can be controlled and understood down to submicroscopic dimensions. Wetting properties of superhydrophobic surfaces, which are useful in various applications, are studied in part III of the thesis.