What are Newtonian and non-Newtonian fluids?

Question

What are Newtonian and non-Newtonian fluids?

Answer 1

Newtonian fluids are fluids that deform indefinitely when introduced to a shearing stress. Most fluids (both liquids and gases) you come in contact with are considered Newtonian.

Non-Newtonian fluids simply don't follow the above, as they exhibit characteristics of both solids and fluids. Silly Putty is one example.

Answer 2

The above answer is correct except he did not mention the mathematical relationship. Fluids that obey Newton's law of viscosity are called Newtonian fluids.

Tau = mu*(gamma) = mu*(dv/dx)

Fluids which satisfy the above equation are called Newtonian fluids, and the ones that don't are called as non-Newtonian fluids.

Tau = Force/Area

mu = Dynamic Viscosity

dv/dx = Velocity gradient across distance in x