Shear stress, velocity gradient, Shear rate, Time of shearing, Newtonian and Non-Newtonian fluid.
The viscosity of a fluid reflects the property of being able to transmit friction forces between particles or to exhibit resistance to shearing flows. For a quantitative characterisation of viscosity of a fluid, the laminar flow is considered when the relative motion between two adjacent layers generates shear stresses. In the case that the magnitude of tangential stress is proportional to velocity gradient on the direction normal to flow, it is said to be a Newtonian behaviour. When the viscosity of the fluid is depending on shear rate, it is called non-Newtonian fluid.The viscosity of real materials can be significantly affected by such variables as shear rate, temperature, pressure, molecular structure, molecular weight and time of shearing.