Physicists Determined That Cats Are a Liquid

Marc-Antoine Fardin, a physicist at Paris Diderot University, was inspired by a post at boredpanda.com called “15 Proofs That Cats Are Liquids” and set out to use the tools of his trade to determine if this is, in fact, true.

Figure from "On the Rheology of Cats": (a) A cat appears as a solid material with a
consistent shape rotating and bouncing, like Silly Putty on short time scales.
(b) At longer time scales, a cat flows and fills an empty wine glass.
(c-d) For older cats, we can also introduce a characteristic time of expansion and
distinguish between liquid (c) and gaseous (d) feline states.

Rheology is the branch of physics that studies the flow of matter. Matter can come in three forms: solid, liquid and gas. Under pressure or stress, solid matter deforms whereas liquid and gas matter flows. Liquid matter is incompressible, whereas gas matter is compressible. Thus, liquids are substances that conform to the shape of their containers (i.e. are fluid) and have constant volume (i.e. are incompressible).

Flow is the process of conforming to the shape of containers and has a set duration for different substances. In rheology, this duration is called the relaxation time. The ability to determine if a substance is a liquid depends then on whether you observe it for longer than its relaxation time. Based on the evidence provided in images, Marc-Antoine determined that cats can, in fact, conform to the shapes of their containers if given enough time. Therefore, cats are liquid.

But this leaves us with additional questions about how cats flow. For one thing, some fluids are more viscous (thicker) at some times and less viscous (runnier) at others. This property is called thixotropy. Do cats exhibit thixotropy? In other words, does the relaxation time of a cat depend on its age? And do they flow with vortices or with laminar flow? A substance flowing with vortices would spin around the container and start to climb of the walls of the container. A substance flowing in a laminar way would calmly follow the outline of their container. Cats may be a fluid that can do both.

Figure from "On the Rheology of Cats": (a) A cat spontaneously rotates in a cylindrical jar.
(b) Normal forces and Weissenberg effect in a young sample of Felis catus.

Clearly, more work needs to be done on this very important question. If you have a cat, you can explore this question with some photographic evidence of your own.


Want to know more? Check this out:

Fardin, M.A. (2014). On the Rheology of Cats. Rheology Bulletin, 83(2):16-17.