by David Moonay
AL
What is Shear Rate and Why is it Important?
Shear rate is the rate at which a fluid is sheared or “worked” during flow. In more technical terms, it is the rate at which fluid layers or laminae move past each other. Shear rate is determined by both the geometry and speed of the flow. If someone quickly rubs a very thin layer of ointment, cream, or lotion on the skin, for example, then the shear rate may be much
higher than if that material is slowly squeezed out of its tube. Figure 1 shows an example of simple shear, in which one plate moves paral- lel to another.
Viscosity is the resistance to flow. In this case, there is some friction between the lay- ers. Therefore, as the upper plate moves with speed or velocity v2
of that motion is transferred downward from layer to layer, until the stationary plate is reached. The stationary plate has speed or velocity v1
equal to zero. The distance between
plates is x, cm. Therefore, the shear rate in this case is
(1) equal to V, cm/s, some
where ̈ is the shear rate, reciprocal seconds, denoted s-1
or 1/s.
Figure 2 shows a force, F, dynes, applied to the top fluid layer. This is translated downward through the layers. The shear stress, τ, is the force per area, dynes/cm2
.
The viscosity, η, is the relationship between the shear stress and the shear rate.
(2) Figure 1 – Example of simple shear, in which one plate moves parallel to another.
Simple liquids, at constant temperature and pressure (and without chemical reactions or phase changes occurring), have a constant viscosity. These are called Newtonian liquids. The viscosity remains constant with chang- ing shear rate. However, most fluids are non-Newtonian, that is, their viscosities are a function of shear rate. Therefore, changing the geometry, such as the distance between the two plates, above, and/or the speed of flow, may significantly change the viscosity. We refer to the “apparent viscosity” or ηA this case.
in Figure 2 – Simple shear between two parallel plates. AMERICAN LABORATORY 35 JUNE/JULY 2017
Shear rate is thus important because it may significantly affect the viscosity of many ma- terials. This, in turn, must be considered when designing pumping, mixing, and spraying systems. It is important in designing various
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