Chapter 1: The Scientific Inquiry

In the realm of modern physics, researchers often grapple with complex concepts like the mass of neutrinos, the incompatibility of quantum field theory and general relativity, and the potential of nuclear fusion as an energy source. While these issues are undoubtedly significant, they don't quite resonate with the everyday curiosities of the general populace. For instance, a popular question that arises is: are cats more akin to liquids or solids?

The initial exploration of this idea can be traced back to a researcher named Tom, who, in 2014, submitted his observations to the Funny Animals section of Bored Panda under the title "15 Proofs That Cats are Liquids." Through a series of 15 experiments involving various cat breeds and container shapes, Tom concluded that cats exhibit a tendency to adapt to the shape of their surroundings—a characteristic typical of both liquids and gases. Additionally, he noted that the volume of cats showed minimal change during these experiments, further aligning them with liquid properties.

Seeking a more rigorous examination of this topic, Marc-Antoin Fardin from the École normale supérieure in Paris published a follow-up study later that same year, titled "On the Rheology of Cats." Fardin's research revealed that cats demonstrate a diverse array of rheological behaviors, making it inappropriate to label them strictly as liquids. His contributions to the whimsical yet profound field of cat physics earned him the 2017 Ig Nobel Prize in Physics.

Section 1.1: Understanding Rheology

Fardin's work, while humorous, invites a serious discussion around the definitions of solid and liquid states by employing the language of modern rheology. This branch of mechanics investigates the behaviors of materials that possess both fluid and solid characteristics. Rheology falls under the broader umbrella of continuum mechanics, which examines the motion of substantial bodies of matter, disregarding their microscopic structures.

Materials can be positioned along a spectrum based on their resistance to shape changes. At one end are ideal Newtonian fluids, which deform indefinitely under applied stress, and at the other are rigid solids that resist any deformation, no matter how forceful the application.

For instance, glaciers, although solid ice, flow over extended periods, behaving like liquid rivers when observed over time. Similarly, when a water balloon is punctured, the water retains the balloon's shape momentarily, suggesting solid-like behavior before it flows away.

To quantify a material's behavior, scientists assign a Deborah number based on the duration of an experiment in relation to the material's relaxation time. A Deborah number greater than one indicates solid-like behavior, while a number less than one suggests liquid-like behavior.

Subsection 1.1.1: Observing Cats in Motion

Fardin conducted a straightforward investigation to determine the relaxation time for cats. His findings, supported by numerical simulations from Cat Bounce, suggest that a cat behaves like a solid when observed for about one second. This aligns with common experiences, such as when a cat falls from a low height and rotates to land on its feet.

Conversely, when observing a kitten settling into a wine glass, it adapts to the shape of the container, indicating liquid-like behavior. Cats typically take no longer than a minute to find and settle into a space, suggesting they exhibit liquid characteristics over timeframes of approximately one minute.

Chapter 2: The Phenomenon of Wetting

Tribology, the study of surfaces in contact, examines whether a liquid can "wet" a solid surface. A liquid is considered to wet a surface if it spreads across it; if surface tension overcomes intermolecular forces, the liquid maintains a droplet shape, signifying poor wetting.

Fardin presents examples of surfaces that repel cats, demonstrating the concept of "superfelidophobia." For instance, a straw basket illustrates a high contact angle, indicating cats struggle to maintain contact with smooth surfaces, while they prefer rough textures like carpets or pillows.

Cats notoriously shy away from water, showcasing a strong aversion to wet surfaces.

... and so forth with further exploration into flow instabilities and the implications of active versus passive fluids.

In summary, while the notion of cats as liquids is largely humorous, it serves as an engaging gateway into significant concepts within continuum mechanics, allowing readers to appreciate the intersection of science and everyday life.