High‐speed optical imaging of liquid film flow and liquid macro‐slip over free surfaces with different surface energies

Abstract

High‐speed optical imaging is introduced as a visualization method to investigate the film‐flow properties of liquids with different surface tensions and viscosities (water, poly‐alpha‐olefin oil, and glycerol) over free rotating surfaces with different surface energies and polarities (steel, and two different diamond‐like‐carbon ‐ DLC coatings, i.e. DLC, and F‐DLC). It was found that the polar surface energy strongly influences the structural dynamics of the liquid film's flow and the film's slip. Namely, a decrease in the polar surface energy results in a less stable film with de‐wetting areas and breakups into streams, as well as in a larger amount of film slip, which was most clearly expressed by the F‐DLC. It was also found that the combination of a high surface tension and a low viscosity provides the largest amount of liquid slip, with the most obvious breakup of the liquid film being observed with water, which clearly exhibits these properties.

Free surface lubrication of rotating cylinders by impacting Newtonian liquid jet

“…Jet impaction can be significantly affected by presence of substrate coating with a liquid film when the film is thick and of low viscosity 2 . Nevertheless, if the liquid film coating is thin and viscous (as was the case in our experiments), the slippage of liquid against the cylinder and between layers of coating and jet‐deposited film can be neglected 25 . Consequently, we do not expect deposition mechanism to differ significantly from the dry surface impingement.…”