2. Methods for estimating liquid viscosity

There is no theoretical basis comparable to that of gases for estimating the viscosity of liquids; all evaluation methods are empirical and have not proved to be very accurate. So, to find out the viscosity of a liquid, it's best to rely on compilations of data or, better still, to carry out experimental measurements. Viscosities of liquids are always higher than those of gases at the same temperature, except at the critical point where the viscosity of the saturated vapor is equal to that of the saturated liquid. As we saw earlier, gas viscosities at atmospheric pressure increase almost linearly with temperature, while liquid viscosities at atmospheric pressure decrease almost logarithmically with temperature. Molecular structure plays an important role in viscosity: at comparable reduced temperatures, the viscosities of polar liquids are higher than those of non-polar liquids such as hydrocarbons,...