Re: Would the semi-liquid form of 'slime' be considered a solid or liquid?

This is a great question!! On a fourth grade level, there are simple definitions of the terms 'solid' and 'liquid' that help determine the phase of slime.

Solid - Substance with a fixed volume and shape
Liquid - Substance with a fixed volume but assumes the shape of the container into which it is poured

By these simple definitions, slime would be a liquid because it will take the shape of its container. However, the source of confusion is most likely because slime has such a high viscosity. In simple terms, this means it flows very slowly when poured. In contrast, water has a relatively low viscosity at room temperature and is easily poured.

Newton is famous for several things, including his definition of liquids and their flow. Basically, fluids fall into two categories; Newtonian and Non-Newtonian. Newtonian fluids are very common and behave like we think liquids should behave. For example, water is a Newtonian fluid. These are generally fluids with low viscosity.

Slime is a Non-Newtonian fluid. Non-Newtonian fluids generally have higher viscosities. This is because the basis of Newton's definitions of fluid behavior relates largely to the shear forces in fluid flow. Non-Newtonian fluids have higher viscosities, therefore more shear force (i.e. resistance to flow).

An interesting thing to show the students is the special properties of this Non-Newtonian fluid. If they press their finger very slowly into the slime, it goes very easily into the fluid. However, if they press very rapidly, the slime will probably bounce back a little. This is because of the special property Non-Newtonian fluids have with respect to the resistance to flow.