A surprising foam discovery could change everyday products

Anyone who has sprayed foam on a surface has likely noticed droplets forming and dripping from the bottom. This happens because foam is made up of tightly packed bubbles separated by thin liquid films, creating a complex network of pathways. Liquid can move through these pathways, either draining out or being absorbed into the foam when it comes into contact with it. Scientists have long believed that this process is controlled by the "absorptive limit," which depends on "osmotic pressure," a measure of the energy change when bubbles are compressed and the contact area between liquid and gas shifts.

Why Old Models Did Not Match Reality

However, this explanation has not matched what researchers observe in real life. Calculations based on osmotic pressure suggest that foam would need to be about a meter tall before liquid begins to drain. In practice, even foams just a few tens of centimeters high can leak easily. This gap between theory and reality has puzzled scientists for years. Since foams are widely used in products ranging from cleaning solutions to pharmaceuticals, understanding how they behave is essential for improving their performance, such as creating foams that resist drainage.

Experiments Reveal a Universal Pattern

The research team, led by Professor Rei Kurita, studied simple foam systems created using different surfactants to produce a range of foam types. They placed these foams between transparent plates and positioned them upright, allowing them to directly observe how liquid moved inside. Their experiments revealed a consistent pattern: the height at which drainage begins is inversely related to the liquid content of the foam, regardless of the type of surfactant or the size of the bubbles. They also calculated an "effective osmotic pressure" for this process, which turned out to be much lower than predictions based on bubble size and surface tension alone.

Bubble Movement Drives Foam Leakage

To better understand what was happening, the researchers recorded video inside the foam. At the point where drainage begins, they saw that liquid was not just flowing through static channels. Instead, it was causing the bubbles themselves to shift and rearrange. This led them to identify "yield stress" as the controlling factor, which is the amount of pressure required to move and reorganize the bubbles. Their model, based on this idea, accurately predicts the foam heights at which drainage occurs.

A New Way to Understand Soft Materials

These findings change the way scientists think about foam drainage. Rather than viewing foam as a fixed structure with liquid flowing through it, it should be seen as a dynamic system where the structure itself can change. The researchers hope this new perspective will lead to deeper insights into soft materials and help guide the design of improved foam-based products.

This work was supported by JSPS KAKENHI Grant Number 20H01874.