Squishy Materials

stances they use in the kitchen and the bathroom are not simple liquids or solids. Everyone is familiar with three states of matter: solids, liquids, and gases. However, creams, shampoo, toothpaste, and ketchup all have properties of both liquids and solids. This paper describes demonstrations and laboratory exercises1 that show intriguing properties of squishy substances, defined as materials that are not unambiguously solid, liquid, or gas. Unlike some areas of physics, the concepts behind squishy materials are understandable even by beginning students. Squishy physics can be used to show physics questions arising from everyday life and to convey the excitement of current research. The activities described in this paper have two goals. The first goal is to show how complex fluids (such as sand or shaving cream) differ from simple liquids (such as water or honey). The key difference is that the effective viscosity of a complex fluid strongly depends on how it is measured. The second goal is to get students to think about how the microscopic properties of a sample relate to the macroscopic properties. We do this by exposing students to the theory of jamming2 so that they get a taste of current research not normally seen in introductory science courses. At Emory, we have incorporated the ideas in this article as a short unit within an undergraduate freshman seminar that covers several topics in science. These activities could easily be adapted for other introductory physics courses, and the treatment can be conceptual or quantitative.