How to measure the thickness of a lubrication film in a pancake bubble with a single snapshot?

摘要

In the in-line bright-field image of a pancake-like bubble, a ring-shaped zone of maximum intensity is visible, called the glare ring. It is due to multiple interactions of light with the bubble interface. In this study, we develop a method to measure the thickness of the lubrication film around a pancake-like bubble translating inside a microchannel, based on the location of this glare ring. By means of ray tracing, a correlation is proposed to relate the film thickness to the location of the glare ring with respect to the bubble edge and to the ratio of refractive indices of the inner and outer phases. This makes the method also applicable to inviscid pancake drops. Additionally, for static bubbles, the method can be used to measure the depth of a microchannel. For moving bubbles, provided the speed of the bubble is also measured, the method can be used to measure surface tension or viscosity. Finally, the method can also be extended to viscous drops, provided the shape function of the interface is adapted. Published by AIP Publishing.