They dare to squeeze into any narrow place. Are cats actually liquid?

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Cat owners know the flexibility of cats, which is particularly vividly reflected in the works of painter Utagawa Kuniyoshi:

The cat's ability to change and adjust its body shape to fit into various containers is indeed beyond our reach. An octopus can easily slip through a very narrow gap or hole, but it is an invertebrate after all - how does a cat, as a vertebrate, do this?

About 15 years ago, on New Year's Day, we spent hours searching for our cat. The fireworks had frightened the poor animal into hiding. We searched all of its favorite places in vain; the cat seemed to have simply vanished. But at one point, we were surprised to see a dark, furry mass crawling out of a long, narrow crack under the fireplace.

It turns out that our cat Tigrou hid in a place that seemed completely impossible to contain. Many people have had similar experiences. In recent years, the "cat is liquid" meme has spread on the Internet and attracted a lot of attention. Marc-Antoine Fardin, a physicist at the University of Paris City and the French National Center for Scientific Research, is one of the people who are attracted by these memes.

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“I spend some time online,” he said in a 2019 TEDx talk, “for research purposes, of course.” In the spring of 2014, Fardan began studying the fluid behavior of cats for scientific purposes—his way of getting away from his job.[1] “This procrastination actually led to something of a success,” he explained in his talk, “and it won me the Ig Nobel Prize in Physics, which is an award for ‘first laugh, then thought.’”

What is Liquid?

States of matter, such as solids and liquids, can be understood in many ways. For example, you may have learned in school that the molecules of solids are tightly packed and fixed in place, while the molecules of liquids are free to move around each other.

However, Fardan's research is based on the science of fluid dynamics , or rheology . In this field, objects with a fixed volume and a fixed shape are considered solids. In contrast, the volume of liquid substances remains constant, but their shape can change. Cats fit the latter criteria: despite their constant volume, they can bend their bodies at will to fit into containers such as cardboard boxes or sinks. Does this mean that cats should be classified as liquids?

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It's not an easy question to answer. "If you wait long enough, eventually everything will flow. That's the motto of rheology," Fardan said in his TEDx talk. For example, solid asphalt on an inclined road will flow very slowly, which can be observed after decades. Solids can also deform if enough pressure is applied. On the other hand, liquids can also behave like solids. For example, ketchup will only flow out of the bottle if the bottle is shaken many times.

Therefore, the previous definition of liquid is not entirely accurate. What is a liquid obviously depends on how long you observe it. Rheologists therefore use a value called the Deborah number to indicate the fluidity of an object. In principle, the smaller the Deborah number, the more liquid the substance is.

The Deborah number depends not only on the observation time (T) but also on the so-called relaxation time (τ), i.e. the time it takes for the liquid to adapt to its shape.

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If you pour water into a cup, it fills up very quickly, so the relaxation time is very short. For a thick liquid like honey, you have to wait longer. By combining the relaxation time with the ratio of the observation time, τ/T, we get the Deborah number. If the Deborah number is less than 1, the substance is considered a liquid. If the Deborah number is greater than or equal to 1, it is considered a solid[2]. The longer the observation time, the smaller the Deborah number and the more fluid the substance appears.

To us, mountains appear unmistakably solid. Their fluid behavior is impossible to observe over the timescale of a human lifetime. But over timescales of millions of years, that changes. In fact, the Deborah Number takes its name from a line in the Old Testament, known as the Song of Deborah, which states: "The mountains melt at the presence of the Lord." The Deborah Number reminds us that whether cats can be classified as liquids—or what the definition of a liquid is—depends on how long you observe them.

What cats and liquids have in common Still, rheological studies can be performed that reveal many of the liquid properties of cats.

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To calculate a cat's Deborah number, you first need to determine their relaxation time. This varies based on the animal's breed, age, and other factors. For example, kittens may have a longer relaxation time because they wiggle around a lot and need several hours to settle down and adjust their bodies to their surroundings.

In his lecture, Fardan pointed out that the shape of the container or environment also matters. For example, a cat may relax faster on its owner's lap than in a transport cage on the way to the veterinarian. This difference does not invalidate the cat as a liquid. The relaxation times of many common liquids also vary depending on the environment. Water forms droplets on hydrophobic surfaces such as Teflon, but spreads easily on other surfaces.

In a study published in Rheology Bulletin in 2014, Fardan proposed that the relaxation time of young adult cats is between 1 second and 1 minute. This estimate can be used to calculate the Deborah number: assuming a cat squeezes into a small cardboard box in 5 seconds and is observed for 1 minute, the Deborah number is De = 0.0833... This value is significantly less than 1: this shows that the cat is clearly exhibiting fluid behavior.

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As Fardan points out in his paper, cats share other properties with liquids. For example, they have a yield stress, which means they will “flow” out of their container only after a certain minimum force is applied. This is similar to ketchup in a plastic bottle, which must be squeezed for the ketchup to flow out.

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In addition, cats, like liquids, will adapt their bodies to the shape of the container they enter, thus filling it completely. Another property that cats share with some liquids is high surface tension, which is particularly evident when they squeeze in or out of small containers. Fardan is also interested in other flow properties of cats, such as whether they can generate turbulence. But he points out in the paper that cats fall into the category of "biologically active substances" that, like bacteria, bird flocks, and fish schools, "have their own dynamics," making them difficult to evaluate in this way.

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“All in all, much work remains to be done, but cats have proven to be a rich model system for rheological studies,” Fardan writes. Biology offers another perspective. From a life science perspective, cats are more like liquids than other creatures, such as humans, because they have flexible (and sometimes missing) collarbones. Once their heads can fit through an opening, the rest of their bodies can follow with ease. That’s how Tigrou was able to “flow” into the narrow gap under our fireplace.

References:

By Manon Bischoff

Translated by tim

Proofreading/tamiya2

Original article/www.scientificamerican.com/article/are-cats-actually-liquid/

This article is based on the Creative Commons License (BY-NC) and is published by tim on Leviathan

The article only reflects the author's views and does not necessarily represent the position of Leviathan