Observation and measurement of bubble separation at liquid-liquid interfaces

Incomplete separation of bubbles at liquid-liquid interfaces during steel casting can lead to subsurface problems in castings that can ultimately result in product defects. An oil-water model was used to study the separation of bubbles at an oil-water interface. It was found that rest times of droplets and bubbles at the interface are quite variable with average rest times a function of chemistry and temperature (due to viscosity and interfacial tension changes) and bubble size. The results of these studies were compared to a study of bubbles separating at a steel-slag interface conducted by X-Ray Fluoroscopy. All of the phenomena noted in the separation of bubbles at an oil-water interface were also documented in high temperature experiments. The low temperature results were repeatable and allowed a better understanding of the factors controlling separation. It is postulated that the mechanism of film rupture after drainage is responsible for the variation in measured rest times and accounts for the fact that observed rest times are significantly greater than observed or calculated drainage times.