Viscosity

Viscosity is the property of matter which is in evidence when the relative motion of parts of any body or substance decays on its being left to itself. The gradual cessation of waves on the sea or of wind in the air, the dying away of sound, the frittering away of the energy of a tuning-fork are illustrations of the effect of viscosity. The property is possessed by all substances, gaseous, liquid, and solid. The kinetic theory of gases gives a very simple explanation of viscosity in fluids, or fluid friction as it is also called. Suppose, for example, that there are two contiguous layers of gas flowing with different speeds. This fact will not prevent the individual molecules diffusing across the interface that separates the two regions. Thus the one layer will on the whole gain momentum, and the other lose it, in the direction of motion of the latter. The tendency will be to an equalisation of momentum of the two regions, and the rate at which this takes place across unit area is the measure of the viscosity. Although the molecules of a liquid have not so great freedom of individual motion as have those of a gas, still it is easy to see that a similar interaction will take place between contiguous finite portions moving relatively to one another. The action will no doubt involve dissociation or the breaking up of groups of molecules which assume other and for the moment more stable configurations. According to Maxwell's theory of the constitution of bodies, the difference between a liquid and a solid is that in the former all the groups of molecules readily break up, while in the latter only a small number do so. But it is evident that, if only one of a vast number of molecular groups so yields to the stresses acting on it, the perfect elasticity of the substance will be destroyed. The substance on recovering will not be able to give back the energy spent in deforming it. Any vibrating solid, such as a tuning-fork or wire undergoing torsional oscillations, loses more or less rapidly its energy of motion. In ordinary circumstances this loss is largely due to the resistance of the air, which is ultimately determined by its viscosity. But even if the vibrations were executed in a perfect vacuum there would still be decay of motion in virtue of the viscosity of the solid material itself. The amount of viscosity will depend upon the number of molecular groupings, which break up under influence of the imposed stresses and assume new configurations of stability. See ELASTICITY, FRICTION, STABILITY. It is interesting to note that the kinetic theory of gases implies, almost of necessity, the existence of viscosity, which is simply the diffusion of momentum.