The Viscosity Concept

Abstract

As everyone knows, liquids vary greatly in the ease with which they flow. The more readily a liquid flows, or the more readily its particles move over each other, the greater its fluidity. And because viscosity is the inverse of fluidity, the greater the fluidity of a liquid, the less its viscosity. Viscosity can be defined as internal fluid resistance. It is the resistance offered by one portion of a liquid flowing over another portion of the same liquid. In order to compare the viscosity of various liquids, we need some sort of a coefficient or unit. In order to arrive at such a unit, we may consider two parallel liquid surfaces of unit area and unit distance apart. If the liquid is to have unit viscosity, unit force must be required to cause the two liquid surfaces to move past each other at a velocity of one cm. per sec. Viscosity is then force per unit area divided by velocity per unit length. In c. g. s. units this becomes M l^-1 t^-1. In other words, for a liquid of unit viscosity, one dyne is necessary to cause planes of the liquid of unit area and unit distance apart to move over each other at unit speed.