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These measurements are usually made with a rotational viscometer or rheometer. Alternatively, the shear rate can be controlled and the stress measured. To generate a flow curve, a sample must be subjected to different shear stresses, and the resulting shear rate measured at each applied stress. Shear rate is the rate of strain change over time. The application of shear stress induces deformation in a sample shear strain quantifies the degree of deformation. Shear stress is a measure of the force acting on a cross-section of a material sample in a direction parallel to its plane. Shear rate (γ) = change in strain/change in time = dγ/dt Shear strain (γ) = deformation/height = x/h Shear viscosity is a function of shear rate, pressure, time and temperature and is mathematically defined as shear stress divided by shear rate (see Figure 1):įigure 1 – Mathematical definitions associated with flow-curve generation. Fluids that move very easily, like water, have low viscosity, while thicker fluids such as paints with high solids content have much higher viscosity. Viscosity quantifies a material’s resistance to flow. Measurement across all relevant conditions is key to optimal use of this simple but effective rheology tool. Flow curves are often measured over a limited shear rate range, which can result in vital behaviors being overlooked as well as inadequate control of the product profile. A flow curve is a graphical representation of how the shear viscosity of a sample changes when it is subjected to different shear rates or shear stresses. This article focuses on the best way to measure flow curves to optimize value. Crucial to this process is flow-curve measurement-the generation of a plot of viscosity as a function of applied shear rate or shear stress. Paints, coatings, foods, personal-care products and inks are examples of products with well-defined flow profiles that benefit from rheological characterization. The flow behavior, or rheology, of a product often defines its value and contributes to processability and ease of manufacture.