Newtonian viscosity কী?

In a plane shear flow $\vect v = v_x(y)\,\vect e_x$, the fluid above a surface $y =$ const exerts on the fluid below it (through a surface element $\dd S$) the tangential force

dragging the slower layer forward and being dragged back in return. The shear stress is $\tau = \eta\,\dd v_x/\dd y$; the coefficient $\eta$ (unit $\mathrm{Pa}\,\mathrm{s}$) is the dynamic viscosity of the fluid, and $\nu = \eta/\rho$ ($\mathrm{m}^{2}/\mathrm{s}$) its kinematic viscosity. A fluid for which $\eta$ does not depend on the shear rate is Newtonian: water, air, oils, alcohol; blood, paint, ketchup and polymer melts are not. Orders of magnitude at $20{}^{\circ}\mathrm{C}$: air $\eta = 1.8 \times 10^{-5}\,\mathrm{Pa}\,\mathrm{s}$ ($\nu = 1.5 \times 10^{-5}\,\mathrm{m}^{2}/\mathrm{s}$), water $1.0 \times 10^{-3}\,\mathrm{Pa}\,\mathrm{s}$ ($\nu = 1.0 \times 10^{-6}\,\mathrm{m}^{2}/\mathrm{s}$), olive oil $0.08\,\mathrm{Pa}\,\mathrm{s}$, glycerol $1.5\,\mathrm{Pa}\,\mathrm{s}$, honey $\sim10\,\mathrm{Pa}\,\mathrm{s}$. Liquids get thinner when heated, gases thicker.