| Summary: | Despite intensive investigations on bubble dynamics, many unknown items are remained concerning bubble coalescence. The objective of the study is to investigate experimentally the characteristics of formation, temporal change in thickness distribution and rupture phenomena of liquid film formed between twin approaching bubbles by the liquid thickness measurement for water applying the improved laser extinction method. The coalescence of horizontally contacting twin bubbles was experimentally studied in constant temperature air–water system. The variation and the distribution of liquid film thickness between twin bubbles were investigated. The experimental parameters were the air flow rate and the measuring position of liquid film thickness. The bubble approach velocity and the close contact duration of bubbles were measured by analyzing the images taken simultaneously with the film thickness measurement. Deformations of bubbles were intensified with increasing bubble approach velocity, and bouncing occurs with an increase in the area of liquid film formation between bubbles. There were two types of liquid film thickness distribution just before coalescence. When bubble coalescence occurs rapidly, the liquid film thickness at near center was thinnest and was increased with distance from center and the thinnest thickness appeared just before coalescence was approximately 1.0 μm. When bouncing occurs, the ring-shaped thinner area in liquid film emerged and shifted from center to the outside of liquid film with increasing approach velocity and close contact duration. The liquid film thickness at commencement of rupture in the ring-wise area with thinner film thickness was approximately 0.3 μm.
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