For shear thickening fluids the apparent viscosity increases with increasing shear rate—the harder the fluid is sheared, the more viscous it becomes. Common examples of this type of fluid include water–corn starch mixture and water–sand mixture 1“quicksand”2. Thus, the difficulty in removing an object from quicksand increases dramatically as the speed of removal increases. The other type of behavior indicated in Fig. 1.7 is that of a Bingham plastic, which is neither a fluid nor a solid. Such material can withstand a finite, nonzero shear stress, yield, the yield stress, without motion 1therefore, it is not a fluid2, but once the yield stress is exceeded it flows like a fluid 1hence, it is not a solid2. Toothpaste and mayonnaise are common examples of Bingham plastic materials. As indicated in the figure in the margin, mayonnaise can sit in a pile on a slice of bread 1the shear stress less than the yield stress2,but it flows smoothly into a thin layer when the knife increases the stress above the yield stress. From Eq. 1.9 it can be readily deduced that the dimensions of viscosity are Thus, in BG units viscosity is given as and in SI units as Values of viscosity for several common liquids and gases are listed in Tables 1.5 through 1.8. A quick glance at these tables reveals the wide variation in viscosity among fluids. Viscosity is only mildly dependent on pressure and the effect of pressure is usually neglected. However, as previously mentioned, and as illustrated in Fig. 1.8, viscosity is very sensitive to temperature. For example, as the temperature of water changes from 60 to the density decreases by less than 1% but the viscosity decreases by about 40%. It is thus clear that particular attention must be given to temperature when determining viscosity. Figure 1.8 shows in more detail how the viscosity varies from fluid to fluid and how for a given fluid it varies with temperature. It is to be noted from this figure that the viscosity of liquids decreases with an increase in temperature, whereas for gases an increase in temperature causes an increase in viscosity. This difference in the effect of temperature on the viscosity of liquids and gases can again be traced back to the difference in molecular structure. The liquid molecules are closely spaced, with strong cohesive forces between molecules, and the resistance to relative motion between adjacent layers of fluid is related to these intermolecular forces. As