Is peanut butter a liquid or a solid? A question that is certainly legitimate and which has recently been debated on the web. For example, according to the Transportation Security Administration, a US government agency, it is a liquid and for this reason it can be transported with a volume of less than 100 millilitres.
So Ted Heindel, University Professor and Director of the Center for Multiphase Flow Research and Education at Iowa State University, wrote on The Conversation website that according to him the definition of “liquid”, associated with peanut butter, makes sense.
This is because any material that continuously flows when a shear force is applied is a fluid. Surely you are used to substances such as water, called in jargon “Newtonian fluids”, or a fluid in which the shear force varies in direct proportion with the stress it exerts on the material, known as shear strain.
For a Newtonian fluid, therefore, the fluid’s resistance to flow, i.e. its viscosity, is constant at a given temperature. When you mix the peanut butter very fast, with more shear force, it becomes more liquid, while when you mix it slowly, it stays stiff. These types of fluids are called “non-Newtonian” (like quicksand).
In general if it can flow and will take the shape of its container, the substance is a liquid (like your cat). There is a difference between liquid and fluid: fluids are divided into gases and liquids. Both gases and liquids can be deformed and poured into containers taking their shape, but gases can be compressed, while liquids cannot (at least not as easily as the former).
