Unstable growth of bubbles from a constriction

TL;DR

This paper investigates the formation and instability of bubbles from a constriction, providing a quasi-static model to predict bubble size and growth dynamics, with implications for microfluidics and chemical screening.

Contribution

It introduces an analytical quasi-static model for bubble formation from a constriction, capturing the instability leading to rapid bubble growth and predicting final bubble size.

Findings

Identified a critical dimensionless number for instability onset.

Predicted bubble size and growth dynamics across different reservoir volumes.

Described rapid bubble expansion beyond the critical pressure.

Abstract

Bubbles and droplets are ubiquitous in many areas of engineering, including microfluidics where they can serve as microreactors for screening of chemical reactions. They are often formed out of a constriction (a microfluidic channel or a cylindrical tube) by blowing a given volume of gas into a liquid phase. It is obviously crucial to be able to control their size, which is not always easy due to the coupling between the volume of the bubble and the gas pressure induced by the Laplace law. In this paper, we examine the size and formation dynamics of soap bubbles blown from a cylindrical tube, which is the paradigm geometry for bubble and droplet formation. To do so, one end of the tube is closed by a soap film, while the other end is connected to a large reservoir of variable volume filled with gas. To inflate the gas in the bubble, we reduce the volume of the reservoir, which mimics air inflation through the lung diaphragm or the flow-rate driven bubble formation in microfluidics geometry such as flow-focusing. As the volume of the reservoir decreases, the gas pressure increases, the soap film curves and takes the form of a spherical cap with an increasingly smaller radius of curvature. This quasi-static process continues until a critical pressure is reached for which the bubble is quasi-hemispherical. Beyond this pressure, the film undergoes a rapid topological transformation and swells very rapidly (in less than a hundred ms) until it reaches its final volume. We describe this instability in particular by showing that this unstable regime appears when a dimensionless number - whose expression we specify - reaches a critical value. Using a quasi-static model that we solve analytically, we predict the bubble growth dynamics and the final height of the bubble produced for any reservoir volume and constriction size.