The formation of air bubbles in liquids appears to be very similar to the formation of liquid droplets from a dripping water tap, which in some sense can be regarded as the inverse process. However, the physics involved in these two processes is actually rather different – while water droplet formation is uniform in size and evenly spaced (a universal process), bubble formation is typically much more random. As an example, one can consider for instance injection of air through an opening into a large tank of liquid – any irregularities in the orifice or injecting with some pulsation, will result in a different pinch-off of the bubbles, so this process is not universal.
Recently, I came across an interesting article related to this topic, published in PNAS by researchers at Massachusetts Institute of Technology (MIT). In this article, the authors explain how bubble pinch-off can also be made universal by confining the bubbles to a narrow space.
The above figure shows the displacement of a partially wetting liquid from a microcapillary tube. As liquid is withdrawn from the right end of the tube at a constant flowrate Q, air enters the tube from the left end and entrains a thin film of glycerol on the tube walls. The dewetting rim starts to recede along the tube axis and grows ahead of the contact line, where liquid, solid and air meet at a non-zero apparent contact angle. As the thin film grows, the bubble neck diameter shrinks in size and ultimately results in pinch-off and bubble formation.
Specifically, it was one of the movies presented in this article that caught my eye, in which a small microbubble acts as a tracer showing the flow direction. In the first stages, the flow is axially dominant but crosses over to a radially dominant flow at later times. This change in flow direction can be observed by a microbubble present in the vicinity of the bubble neck, which is really fascinating to watch.
Although the findings of such research may seem esoteric, controlled generation of drops and bubbles is very desirable in microfluidic applications, like inkjet printing and medical imaging.