Rheology is the study of the flow of matter: mainly liquids but also soft solids or solids under conditions in which they flow rather than deform elasticallyW. R. Schowalter (1978) Mechanics of Non-Newtonian Fluids Pergamon ISBN 0-08021778-8. It applies to substances which have a complex structure, including muds, sludges, suspensions, polymers, many foods, bodily fluids, and other biological materials. The flow of these substances can't be characterized by a single value of viscosity (at a fixe… (More on Rheology)