Micropolar fluids are fluids with micro-structure. These fluids are characterized by two parameters i.e. Characteristics length (depicts the interaction between the micropolar fluid and film gap/geometry) and Coupling number (the ratio of viscous force of relative rotation to Newtonian viscous force).

Whereas

Couple-stress fluids are characterised by only one parameter i.e. couple stress parameter which includes a new material constant responsible for couple stress and the lubricant viscosity. In the absence of micro-structure, simplest theory of couple stresses in fluids arises. The most important effect of couple stresses is to introduce a size-dependent effect that is not predicted by the classical non-polar theories.