December, 202119 CXO NSIGHTSViscosity is an important quality of many consumer and industrial products. For example, customers judge many personal products, such as shower gel and shampoo, by their "feel" and assess the quality of foods, such as ketchup and mayonnaise, by their texture. Similarly, the performance of paints, in their end use, is determined by their viscosities since this affects how easy they are to apply and how they move after application. The likelihood of a paint dripping down a vertical wall will be determined by its viscosity.A fluid is considered Newtonian if its viscosity is independent of the rate at which it is being sheared (e.g., mixed or pumped). Most viscous fluids are non-Newtonian, though. That means to some degree, their viscosity hinges on the rate at which they are being sheared. The majority of these are shear-thinning, meaning their viscosity decreases as the rate of mixing or pumping increases.Viscosity can be measured by several devices, most commonly a Couette, cone-and-plate or capillary viscometer. Their basic operating principle is to measure the force required to move the fluid at a certain velocity. The force can be converted to the shear stress being exerted on the fluid, and the velocity can be related to the shear rate. The dynamic viscosity of the fluid is then the ratio of shear stress to shear rate.The diamond symbols in Figure 1 measure the shear stress versus shear rate, known as a rheogram, for corn syrup measured using a Couette viscometer. The first thing to notice: the relationship between shear stress and shear rate is linear, which means this is a Newtonian fluid.The circles are the plot of viscosity versus shear rate, and because this is a Newtonian fluid, it is constant. This Corn Syrup has a viscosity of 20.7 Pascal-seconds (Pa s) or 20700 centiPoise (cP). For comparison, water (another Newtonian fluid) has a viscosity of 1 cP.Figure 2 shows a rheogram for mayonnaise. In this case, the shear stress data, the diamonds, curve downwards BY RICHARD K. GRENVILLE, PRINCIPAL ENGINEER - MIXING TECHNOLOGY, PHILADELPHIA MIXING SOLUTIONS, AN SPX FLOW BRANDMEASUREMENT AND INTERPRETATION OF VISCOSITY FOR THE PROCESS INDUSTRIES
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