Bridging the molecular and the continuous pictures of wetting dynamics on hydrophilic surfaces

QC 2024-05-23

Abstract

The term ‘wetting' is used by the scientific community when referring to the affinity and the dynamics of liquid films, drops or menisci over solid surfaces. Wetting processes can be observed in everyday life: a water rivulet sliding down a glass window, an oil droplet hovering over a no-stick pan, a drink meniscus traveling up a straw. Given the mundane occurrence of wetting, it may surprise to discover that there is no definitive description of how it occurs in the first place. In the past 50 years the community of fluid dynamics has come up with theoretical models and experiments aimed to demystify the dynamics of contact lines, i.e. the locations in space where liquid, vapor and solid phases meet. One key conclusion of this effort is that wetting dynamics is inherently a multiscale process, whereby flow at all scales is important.

The possibility of investigating the physics of contact lines is limited by the spatial resolution of experiments. In the last two decades a new investigation tool has joined the fray: direct numerical experiments, in the form of Molecular Dynamics simulations. These ‘virtual lenses' enable us to inspect wetting processes with a time and spatial resolution impossible to achieve with experiments. The goal of this thesis is to use Molecular Dynamics simulations to understand how wetting on hydrophilic silica-like surfaces can be modeled using the tools of continuous hydrodynamics, and conversely what effects emerge inherently from the discrete nature of the molecular world.

Molecular simulations sacrifice computational efficiency on the altar of detail and cannot directly reproduce wetting processes occurring at the scale of microns and upward. Accurate meso- and macroscopic models that can incorporate the effects of molecular physics are hence of great importance. The first half of this thesis illustrates the process of parametrizing Phase Field and Volume of Fluid methods with information provided by molecular simulations, as well as the assessment of their physical validity. Contact line dynamics over hydrophilic surfaces where liquid-solid slip is negligible represents a stress-test for continuous hydrodynamics.

The second half of the thesis focuses on molecular scale effects. The local layering and orientation of water molecules close to silica surfaces is found to affect the mobility of contact lines. In particular, molecular motion in the two surface-nearest liquid layers is responsible for a friction asymmetry, whereby hydrophilic surfaces result easier to wet rather than de-wet. The relation between liquid-solid friction liquid viscosity is also studied. It is determined that accurate correlations can be obtained only by accounting for molecular structure at the liquid/wall interface. These results corroborate the view of wetting as an inherently interfacial process and the idea of incorporating molecular-scale physics in its description.

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