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In other materials, stresses are present which can be attributed to the rate of change of the deformation over time. For instance, in a fluid such as water the stresses which arise from shearing the fluid do not depend on the distance the fluid has been sheared rather, they depend on how quickly the shearing occurs. Viscosity is the material property which relates the viscous stresses in a material to the rate of change of a deformation (the strain rate). In the Couette flow, a fluid is trapped between two infinitely large plates, one fixed and one in parallel motion at constant speed u can be important is the calculation of energy loss in sound and shock waves, described by Stokes' law of sound attenuation, since these phenomena involve rapid expansions and compressions.Īlthough it applies to general flows, it is easy to visualize and define in a simple shearing flow, such as a planar Couette flow. It is worth emphasizing that the above expressions are not fundamental laws of nature, but rather definitions of viscosity.I recently was asked about the optimum viscosity for spraying paint. s) provides acceptable spraying, brushing, or roll coating.There is a rule of thumb in the paint industry that a viscosity of approximately 100 cps (1 P, 0.1 Pa I thought that I would extend my answer to other application methods and share the information. However, that viscosity is not measured under just any set of conditions or done with just any viscometer. All aspects of paint flow, including stirring, pumping, transferring, sagging, and application involve shearing actions. Shearing can be envisaged in terms of placing a small amount of liquid in the palm of your hand and smearing it by passing your other hand over the first one. With spraying and brushing, a high shearing stress is applied to the paint. This also is true of reverse roll coating.
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Direct roll coating is thought of as a low shear process (the application roller often just kisses the substrate), but the doctor blade and pre-application rollers may apply high shearing stresses. Shearing of paint breaks up its structure and the viscosity drops to a lower level (often much lower) than the paint had at rest. This is called shear thinning, and the rate of deformation of the paint as it is sheared is called the shear rate (units are reciprocal seconds, s -1). Although the key parameter affecting viscosity is the shear stress, people in the paint industry historically have looked at viscosity as a function of shear rate.