Fluids share the properties of not resisting deformation and the ability to flow (perhaps otherwise described as their ability to take on the shape of their containers). These properties are typically a function of their inability to support a shear stress in static equilibrium. A consequence of this behaviour is Pascal's law which entails the important role of pressure in characterising a fluid's state.
Fluids can be characterised as:
Newtonian fluids; or
- depending on the way stress depends on strain and its derivatives.