Question

Discuss the differences between Newtonian and non-Newtonian fluids in terms of shear stress vs. shear rate characteristics.

Answer 1

Viscosity is the property of a fluid by virtue of its offers resistance to the movement of one layer of fluid over an adjacent layer. Viscosity is the physical property that characterizes the flow resistance of simple fluids. The ratio of shear stress to shear rate is a constant, for a given temperature and pressure, and is defined as the viscosity or coefficient of viscosity.

Newton's Law of viscocity states that the shear stress between adjacent fluid layers is proportional to the velocity gradients between the two layers.

So, based on the law, Shear stress =

The fluid which obeys Newton's law that is, a fluid whose viscosity does not change with the rate of deformation or shear strain is known as Newtonian fluid.

The fluid which do not obeys Newton's law that is, a fluid whose viscosity changes with the rate of deformation or shear strain is known as Non-Newtonian fluid.

Difference between Newtonian and non newtonian fluid

Here Shear velocity is same as shear rate

Newtonian Fluid

Newtonian fluids are fluids having a constant viscosity and a zero shear rate at zero shear stress. That means; the shear rate is directly proportional to shear stress. In other words, the ratio of the shear stress to the shear rate is constant throughout the fluid. Some common liquids and gases such as water and air are regarded as Newtonian fluids. In Newtonian fluids, we can observe a zero shear rate at zero shear stress. That is the shear rate is directly proportional to the shear stress.

Non Newtonian Fluid

Non-Newtonian fluids are the fluids that have a variable viscosity and a variable relationship with shear stress. It is because these fluids do not follow Newton’s law of viscosity. The viscosity of these fluids can change under force. i.e. some liquids such as sauce are runnier when the bottle is shaken. Most fluids we know are non-Newtonian fluids. Many salt solutions, molten polymers and many other fluids can be categorized into this group. Non-Newtonian fluids have a variable relationship between shear rate and shear stress.