Why sponges absorb less than we think

Most of us know foams in everyday life, whether it’s the frothy bubbles on top of a latte, the lather from shampoo, or the thick cushion of suds in a bubble bath. But foams aren’t just fun; they’re workhorses across industries. They clean oil spills, help extract valuable minerals from rocks, keep fires in check, and even make desserts delightfully fluffy.

Scientists have long been fascinated by foams because they are a strange hybrid of matter. They behave a little like gases, a little like liquids, and a little like solids, all at the same time. One of their most useful tricks is absorption: foams can soak up liquids like oil and water. But here’s the catch. For decades, theory predicted that foams should be able to hold far more liquid than they actually do in real-world conditions.

Now, a team of physicists from Tokyo Metropolitan University, Aoi Kaneda and Rei Kurita, has tackled this puzzle head-on. Their study, published in Journal of Colloid and Interface Science, takes a fresh look at why foams have a much lower “soaking limit” than expected. Their answer: it’s not about the equilibrium pressures scientists used to focus on, but about the subtle dance between flowing liquid and the shifting bubbles themselves.

To appreciate their discovery, let’s step back. Foams are made of air bubbles tightly packed inside a liquid, like soap water. This structure gives them special properties. The curved surfaces of the bubbles create pressure differences, a concept known as osmotic pressure. For decades, scientists assumed this osmotic pressure was what controlled how much extra liquid a foam could absorb before it started to drain out.

But there was a glaring problem. When researchers calculated how much liquid a foam should be able to hold based on these equations, the numbers didn’t match reality. A foam that should, on paper, hold liquid up to a meter in height would already start draining at just a few centimeters tall. Imagine buying a sponge that the manufacturer swears can hold a whole bucket of water, only to find it leaking after a single cup.

So what gives?

A new way to look at foam

Kaneda and Kurita worked with different kinds of foams, made from surfactants like TTAB (a lab standard), SDS (a common soap ingredient), and even a household detergent called Charmy sold in Japan. They varied the bubble sizes, liquid content, and foam height, then watched what happened as gravity pulled the liquid downward.

What they found was striking. Instead of behaving like a static porous material, something like a sponge full of fixed holes, the foam was alive with movement. The bubbles weren’t passive. As the liquid drained, the bubbles themselves rearranged, shifting and deforming. This rearrangement changed the balance of forces inside the foam.

In other words, the liquid flow and the bubbles are coupled, they move together. And it’s this kinematic coupling, not just equilibrium pressure, that determines when the foam starts draining.

One of the biggest revelations from the experiments was just how far off the old models were. Classic theory predicted that the osmotic pressure of a typical foam could reach about 2000 pascals (a measure of pressure). But Kaneda and Kurita measured something very different: the effective pressure that actually limits absorption was only about 70 pascals, roughly 30 times smaller.

That’s like expecting a dam to hold back a flood, only to find it can barely contain a garden hose.

Why the huge discrepancy? The key lies in yield stress, the point at which the foam’s bubbles can no longer resist rearranging under stress. Below this threshold, the foam holds together and resists draining. Once the stress surpasses it, the bubbles start to shift, and drainage occurs.

In other words, the foam’s limit is governed not by the tidy math of equilibrium, but by the messy, real-time physics of when its bubble network gives way.

In addition to measuring pressures, the researchers watched the bubbles move. Under non-draining conditions, the bubbles stayed largely in place, holding their structure. But when the foam drained, the bubbles visibly shuffled around, sometimes collapsing into new positions.

This bubble choreography explained why absorption was so limited. It wasn’t that the foam lacked the theoretical capacity to hold more liquid. Rather, the system couldn’t stay stable long enough. The bubbles yielded, the structure rearranged, and the liquid leaked out.

Foams are used everywhere, from firefighting foams that smother blazes, to detergents that lift grease off dirty dishes, to industrial processes like froth flotation, which helps extract copper and other valuable minerals from ore. Knowing the true limits of foam absorption, and understanding why those limits exist, could help engineers design better products. Imagine a firefighting foam that holds water more effectively, or a detergent foam that lasts longer on a greasy surface. Even in medicine, where foamy materials are explored for drug delivery or tissue engineering, these insights could prove transformative.

Beyond foams, the study sheds light on the broader world of soft jammed systems, materials that are dense, squishy, and stuck together, like blood cells in a vessel, emulsions in food, or tissues in the body. These systems all involve dynamic couplings between liquid flow and structural rearrangements, just like foams. By cracking the foam puzzle, Kaneda and Kurita have provided a framework that could ripple outward into biology and materials science.

The beauty of this research is that it reframes how we think about a familiar material. For decades, scientists treated foams like passive sponges, ruled by equilibrium pressures. But Kaneda and Kurita showed that foams are active, dynamic, and messy, more like a crowd of people moving through a busy street than a row of bricks in a wall. On paper, equations can look perfect. But when real-world behavior stubbornly refuses to match theory, that’s often where the most exciting science begins.

So, why can’t foams soak up as much as we thought? Because they’re not just static sponges. They’re dynamic, shifting systems where flowing liquid tugs on bubbles, bubbles push back, and at a certain point, the whole structure gives way.

When you'll watch the froth slip down your coffee cup or rinse a sink full of soap suds, you’ll know there’s a hidden physics drama at play, a delicate tug-of-war between liquid and bubbles. And now, we have a clearer picture of this everyday yet surprisingly complex material.

If you want to learn more, read the original article titled "Absorptive limits of foams governed by kinematic coupling between solution and bubbles" on Journal of Colloid and Interface Science at http://dx.doi.org/10.1016/j.jcis.2025.137746.