**[Terminology] Newtonian Fluid**

**Definition:**the fluid obeying Newton’s laws of viscosity.

**Characteristic:**relationship of D(shear velocity)- τ(shear stress) is straight line, and passes through the origin; the slope is viscosity.

In any small external force, Newtonian fluid can flow. To Newtonian fluid, only viscosity can express its rheological properties.

Most of pure liquids, simple solutions are Newtonian fluids. Such as: water, ethanol and the like.

**Newton's equation:**

τ = ηD

Among:

D=dv/dx

D is also known as velocity gradient.