The interaction of an oil droplet and a gas bubble rising in a quiescent liquid*

Gas flotation for oily-water mixtures is common practice in many industrial cleanup applications. In practice, gas bubbles (typically air) will come into contact with dispersed oil drops leading to the formation of compound drops (gas-filled oil drops) that rise faster. Although this process is widely used in the industry, the details of the interaction and the capture conditions have not been studied in detail. An experimental setup was designed and built to study the interaction between individual air bubbles and oil droplets ascending in a liquid. The motion of the bubble-droplet pair was filmed with a high-speed camera. Many experiments were conducted considering different bubble and droplet sizes, and viscosities of the surrounding liquid.  In most cases, the air bubble and oil drop bounced off of each other rather than coalescing. These observations indicate that the conditions for coalescence fall within a small window of parameters.  We conjecture that the capture conditions are determined by the value of the spreading parameter (a generalization of the well-known spreading parameter) and the Stokes number of the interaction.