The head on a glass of beer is made of many tiny bubbles that form a liquid–gas foam. But the structure is generally short lived. Consider a single isolated bubble when popped: It can produce a spray of droplets. If that happens to one of the many bubbles in a foam, the tiny projectiles could puncture adjacent chambers and produce a chain reaction that causes the entire structure to slowly deflate.
In a 2019 study, Rei Kurita at Tokyo Metropolitan University (TMU) and his graduate student Naoya Yanagisawa observed that droplet penetration is indeed one of the primary mechanisms for foam collapse. Now Yanagisawa, Kurita, and Marie Tani (also at TMU) have established a framework for understanding when and why that droplet formation occurs.
The researchers generate foams using an aqueous soap solution. Such foams are really just a series of soap films surrounding pockets of air. The films always meet in threes along edges called Plateau borders (PBs). High-speed recordings track the motions of the films and their PBs.
The latest experiments revealed what happened when a film at the edge of the foam was punctured: It receded until it reached its PBs. The arriving material caused the PBs to oscillate, as seen in the first movie, and pools of liquid formed at the PB centers.
If the PB oscillation was strong enough to crack one of its remaining two films, both films collapsed, as shown in the second movie. Some of those events generated droplets that flew off and punctured other areas of the foam, thereby sustaining the process.
The behavior that Kurita and his colleagues observed depended on the ratio of the liquid volume to the foam volume, φ. In foams with low φ, the pool of liquid remained at the PB’s center and got pulled along as the films collapsed. The liquid inverted the curvature of the receding front and eventually became a separate droplet, as illustrated on the top row of the figure. But in foams with higher φ, the liquid dispersed quickly, and the fronts collapsed without inversion or droplet formation. The boundary between low and high φ depended on the viscosity of the fluid, the geometry of the foam, and the surfactant used.
The researchers determined that two competing time scales controlled whether a collapsing film would generate droplets: the time for built-up liquid to redistribute itself along the PB and the time for the receding front to become flat. Adjusting those time scales may offer a straightforward way to tune a foam’s stability. (N. Yanagisawa, M. Tani, R. Kurita, Soft Matter 17, 1738, 2021.)
