Passage VII

Introduction

A fluid's viscosity is a measure of its resistance against flowing. In general, a fluid's viscosity decreases as its temperature increases. A fluid is Newtonian if its viscosity depends only on temperature. A fluid is non-Newtonian if its viscosity depends on both temperature and stress (increased pressure is an example of stress).

Some fluids are made by mixing particles with a liquid. Three models of such fluids are discussed below.

Sand-Water Model

Many fluids behave like a mixture of sand and water. When this mixture flows, sand grains rub together. The friction between grains is a source of viscosity. If the grains are coated with water, the water lubricates the grains and reduces friction. Lubrication is most likely if the pores (the spaces between grains) are filled with water.

The average pore volume is lowest when the mixture has settled. If the settled mixture is squeezed, the average pore volume must increase so that the sand grains can move over each other. If the mixture is squeezed rapidly, water does not have time to fill the expanding pores, and the viscosity increases rapidly. If the mixture is squeezed more slowly, the viscosity increases more slowly.

Elongated Particle Model

Many fluids contain particles that are elongated (long and narrow). In the absence of stress, these particles are oriented in no particular direction. Under stress, however the particles in some of these fluids become oriented perpendicular to the direction of their flow, increasing the fluids' viscosity; the particles in other fluids become oriented parallel to the direction of flow, decreasing the fluids' viscosity.

Charged Particle Model

When a fluid is squeezed, its particles rub together and acquire positive or negative charges. Particles that are oppositely charged resist being pulled apart. This resistance increases the viscosity of the fluid. When the fluid is rapidly squeezed, the rate of charged particle production is higher than when the fluid is slowly squeezed.