Non-Newtonian fluid
A non-Newtonian fluid is a fluid in which shear stress is not simply proportional solely to the velocity gradient, perpendicular to the plane of shear. Non-Newtonian fluids may not have a well-defined viscosity.Rheological properties are better studied using tensor valued constitutive equations, which are common in the field of continuum mechanics .
Principle types of non-Newtonian fluid include:
| Type of fluid | Behaviour | Characteristics | Examples |
| Plastic solids | Perfectly plastic | Strain does not result in opposing stress | Ductile metals |
| Bingham plastic | Linear relationship between shear stress and rate of strain once threshold shear stress exceeded | Mud, some colloids | |
| Yield pseudo-plastic | Psuedo-plastic above some threshold shear stress | ||
| Yield dilatent | Dilatent above some threshold shear stress | ||
| Power-law fluids | Pseudo-plastic | Apparent viscosity reducing with rate of shear | Some colloids, clay, milk, gelatine, blood and liquid cement |
| Dilatent | Apparent viscosity increasing with rate of shear | Concentrated solution of sugar in water, suspensions of rice starch | |
| Viscoelastic - having both viscous and elastic properties | Maxwell material | elastic and viscosity>viscous effects | metals, composite materials |
| Oldroyd-B fluid | Linear combination of Maxwell and Newtonian behaviour | Bitumen, dough and nylon | |
| Kelvin material | "Parallel" linear combination of elastic and viscous effects | ||
| Anelastic | Material returns to a well-defined "rest shape" | ||
| Time-dependent viscosity | Rheopectic | Apparent viscosity increases with duration of stress | Some lubricants |
| Thixotropic | Apparent viscosity decreases with duration of stress | Non-drip paints and tomato ketchup | |
| Generalised Newtonian fluids | Stress depends on normal and shear strain rates | Blood | |
See also: Newtonian fluid | rheology | viscosity