# 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 .

Principal 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 | Pseudo-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 |

Dilatant | Apparent viscosity increasing with rate of shear | Concentrated solution of sugar in water, suspensions of rice starch or corn starch, Silly Putty | |

Viscoelastic - having both viscous and elastic properties | Maxwell material | "Series" linear combination of elastic and 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