Combination of laser interferometric and laser extinction methods for precise thickness measurement of liquid film between coalescing twin air bubbles

A combined laser extinction and laser interferometric measurement system was developed for the measurement of the change in thickness of a liquid film formed between colliding twin bubbles. The liquid film thickness during the bubble coalescence process is measured based on the bubble approach velocity and the contact duration. The proposed method affords a ten-fold spatial resolution (∼6 μm) of the liquid film thickness distribution compared to the current laser extinction measurement method. Through the use of the proposed method, it was found that the liquid film in the vicinity of its thinnest area immediately before coalescence was wedge-shaped, with the minimum film thickness immediately before coalescence being 0.3-1.3 μm, which is ∼0.6-μm thinner than the value determined by the current laser extinction measurement method. Greater precision of the proposed method is enabled by its improved spatial resolution.