Development of System for Measuring Bubble Coalescence Time by using a Laser

A new system to measure very short bubble coalescence time was developed using a laser. The system’s reliability was confirmed by comparing the system output with photographic observation by high-speed video camera. The result for aqueous solutions of n-alcohol was in good agreement with the previous data. The system was applied to various kinds of aqueous solutions more dilute than those in previous work where the value of bubble coalescence time was expected to be less than 2 ms. It was revealed that the bubble coalescence time reported in previous work was composed of two parts. The first part was the time length of the contacting period of two bubbles and the second was the time length of the initial stage of a film rupture between the bubbles. The former decreased considerably, even to an undetectably small value, with decrease in concentration, but the latter was constantly at about 0.3 ms regardless of concentration. Effects of bubble age and bubble approach velocity were examined. It was also found that the bubble coalescence time in surfactant solution was significantly influenced in a narrow concentration region as low as ppb order and that the bubbles did not coalesce in solution of higher concentration.