Viscosity of Air and Water

Another material constant of interest in unsaturated soil mechanics is the “Viscosity”. It is an important material constant for analyzing the flow related problems as the viscosities of soil pore air and pore water have a direct bearing on the hydraulic conductivity or, in general, the flow characteristics. Further, the rheology and the soil compressibility also directly depend on the soil pore water characteristics.

Viscosity is defined as the ability of fluid to deform under shear stresses. The resistance of the fluid is due to the friction between inter-particles. Two definitions of viscosity are used in fluid mechanics viz. dynamic viscosity (μ) and kinematic viscosity (ν) based on the conceptual applications. The dynamic or absolute viscosity (N.s/m² or centiPoise) expresses resistance to shearing flows of the fluid. On the other hand, kinematic viscosity is expressed as the ratio of dynamic viscosity to the density of the fluid, which is used in analyzing Reynolds number. The dynamic viscosity of pure water (1cP) at standard temperature is approximately 100 times higher than the pure air (0.02 cP). It is interesting to note that the dynamic viscosity of air increases with temperature contrary to the water where the viscosity decreases as shown in the Fig. 2.12. Therefore, the notion of using warm water for washing clothes can be understood. As the viscosity of the water decreases with temperature, the water will have the more able to penetrate into the smaller pores of the cloth and remove the dirt. The use of detergents would enhance the dirt removal process. The flow characteristics of water through the soils will also be improved with the low viscosity as the conductivity of the water increases. Therefore, the viscosity has a direct bearing on the flow behavior.