Formation and detachment of the enclosing water film as a bubble passes through the water-oil interface

Abstract

The present study aims to reveal characteristics of a bubble as it passes through the interface between two immiscible liquids, namely deionized water and mineral oil. Air was injected into the deionized water to produce bubbles. Initial bubble sizes were varied through using three nozzles with the outlet diameters of 2.0, 3.0 and 4.0 mm respectively. Instantaneous bubble behaviors as the bubble penetrates the water-oil interface were observed using the technique of high-speed photography. A computer code was developed to process the captured images and to extract the bubble shape and trajectory. A comprehensive comparison was implemented among bubbles with different sizes. The results indicate the bubble evolution in the mineral oil is considerably stabilized relative to that in the deionized water. In the mineral oil, the bubble size is small and the bubble shape is regular. As a bubble passes through the water-oil interface, a water film coating the bubble is gradually formed. Larger bubbles entrain more water as it migrates from the deionized water to the mineral oil. With the drainage and detachment of the water film from the bubble, a waterjet is identified inside the bubble, as is observed by all three nozzles. The intensity of the waterjet varies with the bubble size. Large bubble size is responsible for an early detachment of the water film and the lateral disturbance to the bubble motion is suppressed. For the small bubble, it undergoes relatively high resistance of the water-oil interface and a long time span is covered before it passes though the water-oil interface.