CHAPTER XV - VISCOSITY

Abstract

This chapter discusses the flow of liquid or gas in which the velocity of flow is different at different points. This is not an equilibrium state and processes will occur that tend to equalize the velocities of flow. Such processes are called internal friction or viscosity. Just as there is a heat flux from the hotter to the colder parts of a medium in thermal conduction, so in internal friction, the thermal motion of the molecules causes a transfer of momentum from the faster to the slower regions of the flow. Thus, the three phenomena of diffusion, thermal conduction, and viscosity have analogous mechanisms. The viscosity determines the rate of transport of momentum from one point in the flow to another. The chapter explains kinematic viscosity and dynamic viscosity. The kinematic viscosity is a kind of diffusion coefficient for velocity. The chapter then discusses the viscosity of gases and liquids. The viscosity of a gas may be estimated from the fact that internal friction, thermal conduction, and self-diffusion occur in a gas by the same molecular mechanism. The viscosity, like the thermal conductivity, is independent of the pressure of the gas. The viscosity of a liquid usually decreases with increasing temperature; this is reasonable because the relative motion of the molecules becomes easier. In the liquids of low viscosity, such as water, the decrease is appreciable but not very great.