![]() Viscosity vs shear rate) of complex non-Newtonian fluids.Īnother difference between these two properties is the uniqueness of their units. This enables a much more complete characterization of the viscosity fingerprint (i.e. More advanced viscometers rely on measuring shear stress at a given rate of shear. These methods (capillary tubes, funnels…) just measure the time that the fluid takes to flow through a given geometry. In most “kinematic” viscometers, the fluid flow is driven by gravity ( g). Historically, viscosity measurements methods opted for kinematic viscosity since it does not involve measuring a force. In this sense, dynamic viscosity is a more fundamental property while kinematic viscosity is a derived one. For this reason, kinematic viscosity is sometimes referred as diffusivity of momentum in analogy to thermal and mass diffusivities. An alternative way to think about kinematic viscosity is to realize that it has units of diffusivity. In the expression above (Newton’s law of viscosity), dynamic viscosity works as the proportionality constant between the stress F/Aand the rate of deformation or shear rate. The internal resistance of a fluid to flow (dynamic viscosity) implies that there is a force involved in displacing a fluid. Simply put, dynamic viscosity gives you information on the force needed to make the fluid flow at a certain rate, while kinematic viscosity tells how fast the fluid is moving when a certain force is applied. As a result, grasping the physical meaning of these two material properties may not always be so easy. Based on the expression above, two fluids with the same dynamic viscosities can have very different kinematic viscosities depending on density and vice versa. ![]() The relationship between these two properties is quite straightforward.ĭynamic viscosity (also known as absolute viscosity) is the measurement of the fluid’s internal resistance to flow while kinematic viscosity refers to the ratio of dynamic viscosity to density. The two most common types of viscosity are dynamic and kinematic. Viscosity is a fundamental material property when studying fluid flow for any application.
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