Why diluted liquid soap comes out of the dispenser with water instead of foam

The reason is that water and soap do not interact the way we think they do. Water is more liquid, soap is thicker. When water gets in and is squeezed through the dispenser, it doesn't always push the soap out evenly. More often, it looks for the easiest path and pushes through a narrow channel in the thick liquid.

Physicists call this phenomenon "viscous fingers." In a new study, scientists from the University of Chicago have shown that such channels can be slowed down by making the boundary between two liquids smoother. The work is published in Science Advances.

Details

Imagine thick soap as a dense mass inside a bottle and water as a lighter, more fluid liquid. When you press the dispenser, the water doesn't necessarily push all the soap in front of you. It's easier for it to find a weak spot, pass through it and come out.

This is how a thin "tunnel" or branch is formed. From the outside, it looks as if the soap has been diluted badly: instead of foam, a watery stream comes out.

In the laboratory, this effect is studied not in the bottle, but in a very thin gap between two transparent plates. Scientists put a thick liquid in there, and then introduce a more liquid one. If the boundary between the two is sharp, the liquid medium quickly begins to push its way forward with individual "fingers".

The team of Zhaoning Liu, Samar Alkatari, Thomas Wiedebeck and Sidney Nagel tested whether this process could be controlled without changing the composition of the fluids. To do this, the researchers moved one of the plates from side to side. Such a movement slightly "smeared" the boundary between the liquids, making it less sharp.

The result was clear: the stronger and faster they moved the plate, the later these "fingers" appeared, and when they did appear, they grew more slowly. In other words, if the boundary between liquid and dense media isn't as sharp, it's harder for water to make a fast path.

Why it matters

The soap story is just a household example. The same principle is important in industry and the environment.

For example, when one liquid or gas is pumped underground to displace another medium, the more fluid substance may not move in an even front. It may travel through a narrow channel and leave most of the substance in place. This reduces the efficiency of the process.

This behaviour is important in oil extraction, groundwater and underground carbon dioxide storage projects. If a gas or liquid escapes "fingers", it's harder for engineers to control exactly where it goes.

But it's important not to exaggerate. Scientists haven't solved all the problems of oil extraction or CO₂ storage at once. They have shown a fundamental mechanism in the lab: the shape of the boundary between fluids can be changed, and this affects how quickly one fluid breaks through another.

Background

"Viscous fingers" is not a new term. Physicists have long studied such patterns: they appear when a more fluid medium displaces a thicker one in a confined space. Similar branching shapes can be seen in a variety of natural and technical processes.

The new study is important because it suggests a simple way to influence this effect. You don't necessarily have to change the fluids themselves. Sometimes it's enough to change the movement of the walls or the shape of the boundary between them.

For the average person, it explains why diluted soap often behaves so irritatingly. For physicists and engineers, it's another step towards better controlling the movement of liquids where a mistake can cost far more than a ruined dispenser.

Source

Zhaoning Liu, Samar Alqatari, Thomas E. Videbæk, Sidney R. Nagel, "Effect of translational shear on interfacial structure in the viscous fingering instability," Science Advances, 2026.