![]() This liquid is a runny goo until you apply stress to it, and then it suddenly acts like a solid. Oobleck is a mixture of cornflour and water (similar to uncooked custard) named after a substance in a Dr Seuss book. In this case, the sauce’s viscosity decreases and it gets runnier with applied stress. This causes the tomato sauce to become more liquid and you can easily squirt some out. So what do you do? You shake or hit the bottle. You know there is some in there, but when you turn the bottle upside down, nothing comes out. Say you want to get some tomato sauce out of the bottle. Remove the stress (let them sit still or only move them slowly) and they will return to their earlier state. If you apply a force to such fluids (say you hit, shake or jump on them), the sudden application of stress can cause them to get thicker and act like a solid, or in some cases it results in the opposite behaviour and they may get runnier than they were before. 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.Non-Newtonian fluids change their viscosity or flow behaviour under stress. ![]() Lastly, there are at least two other possibilities in the world of viscosity measurements: thixotropy and rheopexy. 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. Their use in drilling muds help prevent blowouts, when slowly pumped underground they can move through a formation uniformly. High molecular weight polymers may undergo this transition particularly those described as associative. 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. This is a less common phenomenon two well-known examples are quicksand and solutions of corn starch. The third condition is shear thickening or dilatancy in which the material becomes increasingly more viscous with increased shear rates. ![]() ![]() Other mixed systems can be pseudoplastic air entrainment in whipped egg whites or cement yields products that flow more easily when subjected to shear. 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. 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. Naturally the viscosity never actually becomes zero that would imply a frictionless sample. 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. In this case, as the shear rate is increased, the viscosity decreases. 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.Ī second type of behavior is one that is fairly common and is referred to as shear thinning or pseudoplastic. Shampoos, liquid soaps and laundry detergents are examples of Newtonian fluids, and within a reasonable range of shear rates, their viscosity remains constant. 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. Non-complex solutions of surfactants usually have this response. When a sample has a constant viscosity versus shear rate, that material is called Newtonian. 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. 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 instrumentation that is typically used consists of a vertical spindle that is immersed into the sample below it. The measurement of viscosity is a fundamental analysis for raw materials and finished products.
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