Explaining foam in the absence of soap: This is a tension gradient

Explaining foam in the absence of soap: This is a tension gradient

Foaming is nature’s way of making beer even more delicious. Not all foams are understood, however, because most of them do not seem to adhere to the descriptive pattern. It is important to understand these different foams because they are often applied in the food processing and petrochemical industries. So holding on A new sheet It tells us that allowing these foams to survive may outweigh the interest in education.

Foaming with reason

Foams and foams are formed when two different liquids are mixed with an air-like gas. But not all liquid additives allow a foam to form, no matter how hard you beat it – I see you, test dark chocolate meringues. Although a foam is actually a complex animal, basic physics is not so difficult.

A foam is basically a collection of air bubbles that are covered by thin films that form a self-supporting network. Thin films are subject to two competing forces. The fluid trapped in the interface will slowly exit due to gravity. This makes the air-bonding layer thinner, which can lead to the foam eventually collapsing. But fluid loss is often reduced (or completely prevented) by two factors.

Let’s look at a simple example. The mixture of soap and water will support a foam. Soap is a surface with a low surface tension, while water is a liquid with a surface tension. The film around the air bubble containing them causes the surface molecules (soaps) to repel each other and expand the width of the film. This sets up a capillary force that pulls the fluid (water, in this case) back into the film.

In some cases, the thinness of the interface creates a gradient in surface tension. Basically, the more water will come out, the higher the soap concentration will be. But soap has a lower surface tension, so there is a difference in surface tension between high and low soap concentration areas. It draws water back into the film. It keeps the bubble stable.

These descriptions rely on surfactants, and the main feature of surfactants is that they do not really want to mix with the liquid in which they are placed. The surface molecules line up the surfaces of the thin films, creating a kind of sandwich structure for the forces that keep the foam stable.

Foaming for no reason

It has become a mystery what foam is found in the mixture of alcohols because they mix completely. Even more confusing, compounds of alkanes (oils of different weights) do not foam. But, mix an alkane like deacene with a curved molecule like toluene and the mixture will foam. None of these liquids act as a surfactant, so how do they support a foam?

A team of French researchers has discovered that this is still below how the liquid surface works. Picture it like this: Imagine a 50/50 combination of two light oils. The question you need to ask is, “What is the composition of the surface?” The immediate and intuitive response should be almost identical at 50/50. If this really happens, the surface tension of the mixture should be the average of the surface tension of the two oils.

As for oils, it really is. But for other compounds (such as alcohol, or toluene and decane), the ratio of the two liquids as a whole differs on the surface. Initially, this was not a bar. But as bubbles form, liquids begin to escape from the films. However, as the thickness of the film changes, so does the texture of the surface. Basically, one liquid leaves the surface faster than another (this causes the surface area to change to volume ratio). This creates a gradient in the surface tension, which pulls the liquid back into the film and stabilizes the foam. However, this gradient can only be formed if the surface tension changes in a straight line with the total ratio of the two fluids.

Significantly, for thin films of these compounds, the surface tension (usually a constant in any compound) depends on the thickness of the film. This is something I did not expect — or at least nanometer-thick images I would have expected. But these images are micrometer thick.

A reason for the foam?

I know I get carried away because I want good physics. Why is foaming important? Well, rebellion and foaming are things that need to be taken into account in industrial processes. An engineer must configure a solution time or change the flow rate to account for the formation of foam (note the difference between pulling a beer and pouring coke). With a better understanding of why and how a foam is formed and stabilized, these processes can be better improved.

Physical Examination Letters, 2020, DOI: 10.1103 / PhysRevLett.125.178002 (About DOIs)

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About the Author: Max Grant

Devoted web lover. Food expert. Hardcore twitter maven. Thinker. Freelance organizer. Social media enthusiast. Creator. Beer buff.

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