Effects of surface wettability on contact line motion in liquid–liquid displacement

Abstract

The behaviors of oil–water–solid interfaces are of basic and applied importance for many fields, such as energy, geochemistry, and biomedical sciences, yet molecular mechanisms are still poorly known. In particular, the traditional adsorption–desorption mechanism based on a single compound does not work to describe the contact line for liquid–liquid–solid systems where the two liquids have similar interactions with the solid surface. Here, we present a series of experiments of water spreading in alkanes, i.e., n-decane, n-dodecane, and n-hexadecane, under various surface wettabilities. The contact line motion is captured by the high-speed camera, allowing us to analyze the friction coefficient, displacement distance and frequency, and activation free energy during the contact line motion via the molecular kinetic theory. We further develop a friction model of contact line motion in liquid–liquid–solid systems by establishing the activation free energy of liquid–liquid–solid systems as a function of the activation free energies of separate one-liquid systems. Results show that the contact line friction coefficients increase with the increasing hydrophobicity (e.g., for n-decane, the coefficient changes from 0.3072 to 2.1967 Pa·s), explained by the increase in the displacement distance and sharp decrease in the displacement frequency of the molecules. Additionally, we find a non-monotonic dependence of the contribution of the surface part to the activation free energy on the wettability, with a maximum percentage of 70%–80% at the intermediate wettability, where the reduced collisions of liquid molecules weaken the molecular adsorption/desorption process. Finally, we find that the correlation coefficient n in our model is affected by the surface wettability, with a maximum at the intermediate wettability, showing a good prediction of the results in the literature. The reason can be attributed to the non-monotonic effects of wettability on the surface part of the activation free energy in two-liquid systems.