Researchers at Tokyo Metropolitan University have uncovered the real reason why liquid is expelled from bubbles, solving a long-standing scientific mystery. Traditional physical models have consistently overestimated how high a bubble must be before liquid begins to leak. By looking closely at the bubble’s behavior, the researchers discovered that the key factor was not simply the movement of liquid through the fixed structure, but the pressure required to rearrange the bubble itself. This finding highlights how important dynamic processes are when studying soft materials.
Anyone who has ever sprayed foam on a surface has noticed that water droplets form and drip from the bottom. This occurs because the bubbles are composed of closely packed bubbles separated by thin liquid films, forming a complex network of channels. Liquid travels through these channels and when it comes into contact with the foam, it is either expelled or absorbed into the foam. Scientists have long believed that this process is controlled by an “absorption limit” that depends on “osmotic pressure.” Osmotic pressure is a measure of the energy change when a gas bubble is compressed and the area of contact between liquid and gas changes.
Why the old model didn’t match reality
However, this explanation does not match what researchers actually observed. Calculations based on osmotic pressure suggest that the bubble height should be approximately 1 meter before fluid begins to drain. In fact, even bubbles only a few tens of centimeters high can easily leak. This gap between theory and reality has puzzled scientists for years. Foam is used in a wide variety of products, from cleaning fluids to pharmaceuticals, so understanding foam behavior is essential to improving foam performance, such as creating foam that is resistant to drainage.
Experiments reveal universal patterns
A research team led by Professor Rei Kurita studied simple foam systems created using different surfactants to produce different types of foam. They placed these foams between transparent plates and stood them upright so they could directly observe how the liquid moved inside them. Their experiments revealed a consistent pattern in which the height at which drainage begins is inversely proportional to the liquid content of the foam, regardless of surfactant type or bubble size. They also calculated the “effective osmotic pressure” for this process, which turned out to be much lower than predicted based on bubble size and surface tension alone.
Foam leakage occurs due to the movement of air bubbles.
To better understand what was happening, the researchers recorded video inside the foam. At the point where drainage begins, we know that the liquid is not simply flowing through a static channel. Instead, the bubble itself was moving and repositioning itself. From this, they identified “yield stress” as the controlling factor. This is the amount of pressure required to move and reorganize the bubbles. Their model, based on this idea, accurately predicts the foam height at which drainage occurs.
A new way to understand soft materials
These discoveries change the way scientists think about foam drainage. Rather than viewing foam as a fixed structure through which liquid flows, we should view it as a dynamic system in which the structure itself changes. The researchers hope this new perspective will lead to deeper insights into soft materials and help in the design of improved foam-based products.
This research was supported by JSPS KAKENHI Grant Number 20H01874.
