The definition of viscosity. Consider a fluid located between two parallel plates with distance h and velocity difference v. The shear force, F, divided by the contact area, A, between the liquid and the plate is the shear stress, F/A = τ. The shear rate is the difference in velocity between the different fluid layers thus the shear rate is γ = v/h. The ratio of shear stress and shear rate is the viscosity, η, and is a material property of the fluid. If the ratio of shear rate and shear stress is constant, as for plasma, we call the fluid Newtonian. Blood has a nonlinear relation between shear rate and shear stress.
KeywordsShear Shear force Shear rate Blood viscosity Newtonian fluid
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