# Are Cats a Fluid? Thinking (and Learning) about CFD Through Humor

The internet has uncovered some intriguing evidence on the rheological state of a common household item. How do physical constants like Deborah’s number, adhesion, relaxation time and catpillary number apply in unusual situations? I came upon this topic from the Improbable Research Podcast and the pictures are from the linked rheology bulletin. Welcome to the internet and read on to learn some CFD terminology as applied to an interesting and ridiculous question.

Deborah Number: Given enough time, everything flows. The ratio of relaxation time to the time of observation can be used to determine if something behaves as a gas, liquid or solid in your timescales of interest. Do older cats have a higher relaxation time? Is the question of whether cats flow reminiscent of the wave-particle duality of light? If so, is it of equal to or greater import?

Solid-Liquid-Gas Continuum: Solids that deform under stress vs liquids that flow to fill a container. Clearly the full spectrum can be observed in certain specimens.

Capillary Bridge: Observed in extensional rheometry experiments of test samples (see 2a). The subject is suspended between arbitrary rigid body surfaces. It remains to be seen how important surface tension and transient strain-hardening are to these CATBER (Capillary thinning and breakup extensional rheometer) experiments and other awesome words.

Lotus Effect: The substrate pictured in 2c seems to exhibit extreme felidaphobicity, showing high contact angles on the outer surface. Suggests possible applications in the tops of tables and desks.

Yield Stress: The ketchup in a bottle effect is observed (2c) due to the cat being below its yield stress. This is distinct from the free surface flow effects observed above.

Reynolds-Weissenberg Number: The ratio of the relaxation time vs the rate of deformation. As this number increases, secondary chaotic flows emerge. For simple, non-viscous fluids this is Turbulence and depends primarily on inertia. For biologically active materials, their rates of deformation can be dependent on its environment.

Don’t forget that you can always perform your own experiments in silico, without harming any cats, using ANSYS Fluent. In the material properties of a fluid you have a variety of viscosity and turbulence models to simulate all of the non-Newtonian felines that you desire. Happy new year!

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