Measurement of Instability of Thin Liquid Films by Synchronized Tri-wavelength Reflection Interferometry Microscope

Film thickness measurement of unstable thin liquid films (TLFs) remains a challenge due to the difficulty in determining the order of fringes prior to the film rupture. In the present work, a synchronized tri-wavelength reflection interferometry microscope (STRIM) was developed and employed to determine the spatiotemporal thickness profiles of the TLFs between air bubbles and various hydrophobic surfaces in 10^-2 M NaCl solutions. Both accuracy and precision of film thickness measurements were found to be better than 3 nm over the range of 0-1 μm. It was found that when the radii of air bubbles were in the range 0.71-0.88 mm, the critical rupture thicknesses of the wetting films formed on hydrophobic quartz surfaces having water contact angles of 95° scattered over a range of 57-335 nm with a medium rupture thickness of 122 nm. For smaller air bubbles with radii of 0.13-0.26 mm, the critical rupture thicknesses were much more narrowly distributed with a medium rupture thickness of 27 nm. The result obtained with the TLFs between two air bubbles, i.e., foam film, showed that the critical rupture thickness was increased from 25 to 40 nm, when the sizes of air bubbles were increased from 220 to 960 μm. Compared to rupture thickness of the foam film, the critical rupture thickness of the TLF between an air bubble and a dodecane droplet was smaller, indicating that the film rupture might be related to the hydrophobicity of interacting surfaces. In addition to attractive surface forces, both wave motions and gas molecules in TLF might be associated with the film rupture.