In other materials, stresses are present which can be attributed to the rate of change of the deformation over time. Stresses which can be attributed to the deformation of a material from some rest state are called elastic stresses.
For instance, if the material were a simple spring, the answer would be given by Hooke's law, which says that the force experienced by a spring is proportional to the distance displaced from equilibrium. In materials science and engineering, one is often interested in understanding the forces, or stresses, involved in the deformation of a material. In a general parallel flow, the shear stress is proportional to the gradient of the velocity.
The word 'viscosity' is derived from the Latin ' viscum', meaning mistletoe and also a viscous glue made from mistletoe berries. Given two known temperature-viscosity points, the viscosity of a liquid can be calculated for a target temperature. The viscosity of a liquid as a function of temperature can be approximated with the Andrade correlation. A fluid with a relatively high viscosity, such as pitch, may appear to be a solid.Ĭalculate the viscosity at a given temperature, when the viscosity is known at two temperatures. Otherwise, the second law of thermodynamics requires all fluids to have positive viscosity such fluids are technically said to be viscous or viscid. Zero viscosity is observed only at very low temperatures in superfluids. This is because a force is required to overcome the friction between the layers of the fluid which are in relative motion: the strength of this force is proportional to the viscosity.Ī fluid that has no resistance to shear stress is known as an ideal or inviscid fluid.
In such a case, experiments show that some stress (such as a pressure difference between the two ends of the tube) is needed to sustain the flow through the tube. For instance, when a fluid is forced through a tube, it flows more quickly near the tube's axis than near its walls. Viscosity can be conceptualized as quantifying the frictional force that arises between adjacent layers of fluid that are in relative motion.