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The measurement of viscosity is a fundamental analysis for raw materials and finished products. The instrumentation that is typically used consists of a vertical spindle that is immersed into the sample below it. Activation of the instruments motor causes the spindle to rotate within that sample; the resistance to that movement is a measure of the inherent viscosity. The rotation rate can be varied to determine viscosity under different shear rates, changes (or uniformity) in viscosity over a range of shear rates defines three different flow behaviors that are commonly found in our industry.
When a sample has a constant viscosity versus shear rate, that material is called Newtonian. Non-complex solutions of surfactants usually have this response. Plotted on a graph as shear rate versus shear stress, a Newtonian fluid's slope is constant; viscosity is defined as shear stress divided by shear rate, so a non-varying slope yields the same value throughout the range of measurement. Shampoos, liquid soaps and laundry detergents are examples of Newtonian fluids, and within a reasonable range of shear rates, their viscosity remains constant. Of course, it may be possible to find an extremely low or high shear rate where that may not be true, but those conditions would almost certainly fall outside of normal use limits.
A second type of behavior is one that is fairly common and is referred to as shear thinning or pseudoplastic. In this case, as the shear rate is increased, the viscosity decreases. Plotted on a graph, the slope becomes increasingly parallel to the shear rate axis as the division of shear stress by shear rate approaches zero. Naturally the viscosity never actually becomes zero; that would imply a frictionless sample. Shear thinning is very common for emulsions such as lotions, creams and food products where droplets of an emulsified internal phase are dispersed in the predominant external phase. Typically the internal phase is an oil such as a triglyceride, ester, or other fatty material, and the external phase is water and/or water soluble components. Other mixed systems can be pseudoplastic; air entrainment in whipped egg whites or cement yields products that flow more easily when subjected to shear.
The third condition is shear thickening or dilatancy in which the material becomes increasingly more viscous with increased shear rates. This is a less common phenomenon; two well-known examples are quicksand and solutions of corn starch. For both materials, it is rather easy to move (or sink) through the solution slowly, however quick jerky movements are hindered as viscosity builds rapidly. High molecular weight polymers may undergo this transition particularly those described as associative. Their use in drilling muds help prevent blowouts, when slowly pumped underground they can move through a formation uniformly. Should the drill encounter trapped gas that is under high pressure the sudden increase in fluid movement causes the polymeric solution to thicken dramatically, reducing the potential for an explosive condition.
Lastly, there are at least two other possibilities in the world of viscosity measurements: thixotropy and rheopexy. These are behaviors that are encountered when the amount of time that a sample is subjected to a constant shear rate is considered, but that’s a discussion for another time.