About 15 years ago on New Year’s Day, we spent hours looking for our cat. The fireworks scared the poor animal so much that he hid somewhere. We searched for all of our favorite places, but to no avail. The cat seems to have disappeared. But at one point we were surprised to see something black and fluffy crawling out of a long, narrow opening under the fireplace. Our cat, Tigre, was hiding in a space that was apparently too small for this creature.
Many others have made similar observations. A meme calling cats liquid has been circulating online for several years. They then caught the attention of physicist Marc-Antoine Fardin of the Jacques Monod Institute, now at the Université de Ville de Paris and the French National Center for Scientific Research. “I spend some time on the internet,” he said in a 2019 TEDx talk, “for research purposes, of course.” In the spring of 2014, Fardin began scientifically studying the fluid behavior of cats. This was a pastime that allowed him to avoid his real job. “This procrastination actually led to some success,” he explained in his talk. “For this, I received the Ig Nobel Prize in Physics, which is given to research that not only makes you think but also makes you laugh.”
There are multiple ways to think about states of matter such as liquids and solids. For example, you may have learned in school that molecules in solids are tightly packed into fixed positions, whereas molecules in liquids are more free to move around each other.
About supporting science journalism
If you enjoyed this article, please consider supporting our award-winning journalism. Currently subscribing. By subscribing, you help ensure future generations of impactful stories about the discoveries and ideas that shape the world today.
But Fardin’s research is based on the science of fluid mechanics, or rheology. In this field, objects with constant volume and fixed shape are solids. In contrast, liquid substances do not change volume but can change shape. The latter criterion seems to apply to cats. Despite having a constant volume, cats can bend as they please to fill containers such as cardboard boxes or sinks. Does that mean cats should be classified as liquids?
Answering that question is not so easy. “If you wait long enough, everything will eventually flow. That’s the motto of rheology,” Fardin said in his TEDx talk. For example, solid asphalt on a slope continues to flow very slowly, and this may be observed after several years or even decades. Solids can also deform if enough pressure is applied. On the other hand, liquids can also have the properties of solids. For example, ketchup will flow out only after shaking an open glass bottle several times.
what teeth liquid?
Therefore, the definition of liquid given so far is not completely valid. What is a liquid obviously depends on how long you observe something. Therefore, rheologists use a value called the Deborah number (De), which depends in part on the observation time (T), to indicate how liquid an object is. As a general rule, the lower the Deborah number, the more liquid the substance is.
In addition to observation time,, The Deborah number also depends on the time it takes for the fluid to adapt its shape, the so-called relaxation time (τ). When you pour water into the glass, it fills up quickly, so the relaxation time is very short. However, for viscous liquids such as honey, you may need to wait longer. By setting the relaxation time and observation time as a ratio, τ⁄T, Get your Deborah number. For values less than 1, the material is considered liquid. However, if their Deborah number is greater than or equal to 1, they are said to be solid.
The longer you look at something, the smaller the Deborah number becomes and the more fluid it appears. Mountains are definitely strong for us. Flow behavior cannot be detected during human lifetime. But after millions of years, things change. In fact, the name Deborah Number comes from an Old Testament passage known as the “Song of Deborah.” (Translations vary, but the King James Bible says, “The mountains melted before the Lord.”)
The big picture: Deborah’s number reminds us that we don’t know if cats qualify as liquids, or what exactly liquids are. It depends on how long you observe your cat.
Cats and liquids have a lot in common
Nevertheless, it is possible to perform rheological studies and these reveal many fluid properties in cats. To calculate the cat’s Deborah number, you need to determine the cat’s relaxation time. This varies from animal to animal depending on breed, age, etc. For example, young kittens are often fidgety, so they may need more time to relax. It can take hours for them to settle down and adapt their shape to the environment.

Cats like to occupy as much space as possible in small containers.
In his lecture, Fardin pointed out that the shape of the container and environment is also important. For example, a cat may relax more quickly in its owner’s lap than in a carrier that takes it to the vet.
This variation rarely disqualifies a cat from the liquid state. Many familiar fluids have the characteristic that their relaxation time changes depending on the environment. Water forms droplets on water-repellent surfaces such as Teflon, but spreads easily on other surfaces.
Works published in magazines rheology bulletin In 2014, Fardin suggested that the relaxation time for young adult cats is between 1 second and 1 minute. This estimate allows us to calculate Deborah’s number. For example, if a cat pushes itself into a small cardboard box within 5 seconds and is observed for 1 minute; de = 0.0833…. This is significantly less than 1, so the cat clearly exhibits fluid behavior.
As Fardin pointed out in his paper, cats share other properties with liquids. For example, they have a yield stress, meaning they require a minimum amount of force to be applied before they flow out of the container. The same goes for ketchup in plastic bottles, which need to be squeezed out. Additionally, like liquids, cats adapt their bodies to the container they enter, filling it completely. Another characteristic cats share with some liquids is high surface tension. This is affected when pushing or extruding into small containers.
Fardin was also interested in other flow properties of cats, such as whether cats can induce turbulence. But cats are difficult to assess in this way because, along with bacteria, herds, and herds, they belong to the class of “biologically active substances” and have “their own dynamics,” he noted in the paper.
“In conclusion, cats have proven to be a rich model system for rheological studies, although much work remains to be done,” Fardin wrote.
Biology offers another perspective on this issue. From a life science perspective, these animals are much more fluid-like than other creatures, such as humans, because they have movable (and sometimes lack) collarbones. Once the head fits into the opening, the rest of the body can easily follow. So Tigrue drifted into the narrow gap under the fireplace.
This article was first published Wissenschaft spectrum Reprinted with permission.