Nanobubbles and nanodrops

In this thesis we address the behaviour of so-called surface nanobubbles. These nanobubbles behave different from ordinary (macroscopic) bubbles in several ways: (i) The gas-side contact angles of surface nanobubbles are lower than those of regular bubbles (i.e. nanobubbles are flatter), (ii) Nanobubbles can resist high tensile stresses, and most importantly: (iii) The lifetime of surface nanobubbles is several orders of magnitude greater than expected. In this thesis we combine numerical techniques (molecular dynamics) and theoretical efforts to explain these properties of surface nanobubbles. Using the same techniques we also conduct a study on more general capillary problems. We clarify conceptual questions that arise when dealing with capillary phenomena, even on the classical manifestations such as Young's law. Additionally, we performed measurements of line tension. Although the existence of line tension has been predicted since the 19th century by Gibbs, it remains elusive due to its extremely weak effect compared to surface tension. Using molecular dynamics, we can directly and accurately measure the line tension of a simple fluid in contact with a solid and we also provide a geometric interpretation of the origin of this line tension. Molecular dynamics also allows to study elastocapillarity, the wetting of soft elastic solids. When a droplet (or bubble) is brought into contact with a solid the effects of the solid on the droplet are well-known, but using molecular dynamics and theoretical approaches we provide insight into how the liquid affects the solid. Especially for soft solids (such as gels) this effect is pronounced. Finally, we investigate the dynamic spreading behaviour of a droplet that is brought into contact with a solid. After initial contact, the droplet starts spreading and the speed with which this happens can be predicted by scaling laws. We provide molecular dynamics and experimental results to validate these scaling laws