The internet is full of cats. One click and you are able to watch thousands of videos of cats doing cute, funny or weird things. Every day hundreds of photographs are uploaded showing cats in every imaginable situation.
No matter if you are a cat person or not, chances are high that you know this phenomenon: Cats that look as if they were liquid when lying in a container.
While looking at these photos one question arises: What state of matter are cats? They appear liquid when they lie in a container. On the other hand they look pretty solid when they walk around. What are cats? Liquid, solid, gaseous?
If we want to answer the question, if cats are liquid or not, we have to take a look at a subfield of physics. Rheology is the science that deals with the deformation and flow behaviour of matter.
The state in which an object retains its volume and shape.
The state in which an objet retains its volume and adapts its shape to the given space
The state in which an objet expands its volume depending on how much volume is available
As mentioned above, cats seem to have several states of matter. In rheology we have the possibility to describe such a behaviour with the Deborah number “De” – To define the Deborah number we need two pieces of information.
We need the observation time which we define with . Usually is the duration of the experiment.
We need the relaxation time which can have different origins. This depends on the flow characteristics of the observed object. Let us define the relaxation time with . Based on Marc-Antoine Fardins research we can say that of cats is between 1 second and 1 minute.
If the Deborah number is greater than 1 the material is solid. The smaller the Deborah number the more fluid the material is.
Currently we assume that matter is entirely liquid, solid or gaseous, but this is not the case. Liquids can also contain solid parts and vice versa. More complex liquids require a good understanding of chemistry and biology.
At this point it gets complicated and further formulas of physics would go beyond the scope of this project. If you are interested in a physical correct explanation, please refer to the article from Marc-Antoine Fardin.
The relaxation time can take on a higher dimensionality than just time. Usually the time scale is considered as a contribution to viscosity. When cats are deformed on their principal axis, it seems that they relax more easily. That suggests that the extensional time is smaller than the shear time.
Because, flows of cats are usually free surface flows, the surface tension between the cat and its surrounding medium can be important.
The wetting and general tribology of cats has not progressed enough to give a definitive answer to the capillary dependence of the catlike relaxation time. It seems that cats are superfelidaphobic. Due to this property it is possible to observe the lotus effect on a cat. This behaviour is usually distinguished from the yield stress. It is also possible that cats are not able to flow because they are below the yield stress.
As mentioned above, if the Deborah number is small a material is flowing. If the rate of deformation increases, secondary flows emerge and eventually become chaotic.
Simple fluids like water are passive, they continue to move or deform so long as there is a power inducing them to do so. Since cats have their own motive power, they are active materials. Therefore it can be difficult to define the rate of deformation.
Some questions remain unsolved. Why are cats the way they are? Are they some kind of super-fluid? I hope this short-story has given you something to think about. Nevertheless, whether liquid or solid, cats carry on to be the true heroes of the Internet.
If you can’t get enough about this topic, I have added some follow up links.