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 his favorite places without success. The feline seemed to have disappeared. But at some point, we were surprised to see something black and smooth crawling out of a long, narrow opening under our fireplace. Tigrou, our house cat, was apparently hiding in a space that seemed too narrow for the creature to fit.
Many other people have made similar observations. Referring to memes cats as liquids they have been circulating online for several years. And they caught the eye of the physicist Marc-Antoine Fardin of the Jacques Monod Institute, now at the University of the City of Paris and the French National Center for Scientific Research. “I spend some time on the Internet,” he said 2019 TEDx talk“for research purposes, of course.” Fardin started in the spring of 2014 to scientifically study the fluid behavior of cats—a pastime that allowed him to avoid his real job. “This delay actually led to some success,” he explained in his speech. “It won me the Ig Nobel Prize in Physics, which rewards research that makes you think as well as laugh.”
There is more than one way to think of states of matter, such as liquids and solids. For example, you learned at school that inside a solid the molecules are packed together in fixed positions, while those in a liquid move around each other more freely.
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But Fardin’s research is based on the science of fluid dynamics, or rheology. In this field, objects with constant volume and fixed shape are solid. In contrast, the volume of liquid substances remains the same, but their shape can change. The latter criterion seems to apply to cats: despite their constant volume, they can bend at will to fill a container such as a cardboard box or sink. That means cats should be classified as liquids, right?
The question is not so easy to answer. “If we wait long enough, eventually everything flows. That is the motto of rheology”, said Fardin in his TEDx talk. For example, the strong asphalt of a sloping road remains very slow, which can be seen after several years or decades. Even solids can be deformed if enough pressure is applied to them. On the other hand, liquids can also have solid properties. Ketchup, for example, flows out of an opened glass bottle after being shaken several times.
what It is A liquid?
The definition of liquid given before is therefore not completely valid. What is liquid clearly depends on how long you observe something. Rheologists therefore use a value called the Deborah number (De), which depends in part on the observation time (T), to indicate how fluid an object is. In principle, the lower the Deborah number, the more liquid the substance is.
In addition to observation time, The Deborah number also depends on the so-called relaxation time (τ), which is the time it takes for a fluid to adjust its shape. If you pour water into a glass, it fills up very quickly, so the time to relax is very short. With a viscous liquid like honey, however, you have to wait longer. By setting the ratio of relaxation time to observation time t⁄T, You will get Deborah’s number. For values below 1, materials are considered liquid. If Deborah is greater than the number or equal to 1however, they seem to be strong.
The longer you observe something, the smaller the Deborah number becomes and the lighter it appears. Mountains are undeniably strong for us. During human life, no flow behavior can be detected. But over millions of years, this changes. In fact, the name of the number Deborah comes from a line in a section of the Old Testament known as “The Song of Deborah.” (Although translations vary, the King James Version reads: “The mountains melted before the Lord.”)
The big picture: Deborah’s number reminds us that it’s unclear whether cats qualify as liquid—or exactly what liquid is—because that depends on how long you observe them.
Cats and liquids share many characteristics
However, rheological studies can be performed, which reveal many of the liquid properties of cats. To calculate the number of Deborah cats, you need to determine their relaxation time. This varies from animal to animal, depending on breed, age, etc. Young cats, for example, may have a longer relaxation time because they move around a lot. They can take hours to settle and adjust their shape to the environment.
Cats like to take up as much space as possible in small containers.
Fardin stated in his speech that the shape of the container or environment is also important. For example, cats can relax more quickly on their owner’s lap than in a carrier that takes them to the vet.
This variability hardly disqualified cats from the liquid state. Many popular fluids have relaxation times that vary with the environment. It forms water droplets on repellent surfaces such as Teflon, while spreading easily on other surfaces.
In work published in the journal Rheology Bulletin In 2014, Fardin proposed that the relaxation time of young adult cats is between one second and one minute. This estimate allows the Deborah number to be calculated: for example, if a cat squeezes into a small cardboard box for five seconds and is observed for one minute, then Of = 0.0833…. That is significantly less than 1: the cat clearly shows fluid behavior.
As Fardin noted in his paper, cats share other properties with liquids. For example, they have a holding tension, meaning that a minimum amount of force must be applied before they exit a container. The same goes for ketchup in a plastic bottle, which needs to be squeezed. Also, cats, like a fluid, adapt their bodies to the container they enter, so that they fill it completely. Another characteristic cats share with some liquids is their high surface tension, which comes into play when you put them in or out of a small container.
Fardin was also interested in other flow properties of the cats, such as whether they created turbulence. But cats, he noted in his article, belong to a category of “biologically active material” along with bacteria, flocks and schools, which have “their own motivational power” and are therefore difficult to evaluate as such.
“Consequently, much more work remains ahead, but cats are proving to be a rich model system for rheological research,” Fardin wrote.
Biology offers a different approach to this question. From a life science perspective, these animals resemble a liquid—much more so than other creatures like humans—because of their mobile (and sometimes missing) collarbones. Once the head fits through the opening, the rest of the body can easily follow. Thus Tigrou flowed into the narrow gap under our fireplace.
This article originally appeared the spectrum of science and reproduced with permission.
