Coalescence of Two Equal-Sized Bubbles with Air Injection

Abstract

This study investigates the coalescence of two equal-sized bubbles with air injection in distilled water experimentally, numerically, and theoretically. The neck expansion is the most prominent characteristic within a few milliseconds after bubbles coalesce. The effects of needle configurations and air injection flow rates are mainly discussed. Air injection during bubble coalescence is identified as an additional driving force for neck expansion in addition to the capillary pressure. The neck expansion time increases linearly with the bubble size at contact while decreasing with the air injection flow rate. The dimensionless neck expansion time follows a consistent linear decrease with the air injection speed. A novel theoretical expression for the dimensionless neck radius is derived, by quantifying the air injection effects as a multiplier (α) of the Laplace pressure at the neck. α demonstrates a consistent linear increase in air injection speed at all the needle configurations and flow rates. Moreover, numerical results of velocity vectors reveal the mechanism of air injection promoting neck expansion.