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