8/16/2023 0 Comments Viscosity![]() ![]() Capillary viscometer and flow cups (orifice viscometers) 1 These two ways of expressing viscosity can be interconverted by considering the fluid’s density, multiplying the kinematic viscosity by the density of the fluid to obtain the absolute viscosity, or dividing the absolute viscosity by the density to obtain the kinematic viscosity. The absolute or dynamic viscosity is determined by observing the fluid’s resistance to flow under a controlled external force, while the kinematic viscosity is determined by observing the fluid’s flow under the action of gravity. Considering the different viscometry methodologies, there are two distinct ways to express viscosity, absolute viscosity or kinematic viscosity. Based on the relative motion and geometry of the equipment, several types of viscometer, as well as their accuracies and target applications can be defined. 6 The relative motion between the fluid and the surfaces of the measuring object causes drag, allowing viscosity to be measured. In either case, the flow conditions must be maintained on the laminar regime. Viscosity determinations can be performed based on a number of methods, where the measuring object stays stationary and the fluid passes through it, or the fluid is stationary and the measuring object moved through it. Furthermore, viscosity should not be perceived as an absolute value, but rather a function of external factors such as temperature, shear and pressure, all of which affect viscosity, and thus, the technological and organoleptic properties of a food product. This property is highly dependent on the intrinsic characteristics of a material such as molecular composition, solute concentration, structural organization and density. In practical terms, a fluid with a higher viscosity would flow more slowly and appear thicker than a fluid with lower viscosity (Figure 1). For a more in-depth understanding of these phenomena and its implications, additional reading is recommended. However, the majority of fluids do not behave this way, being called non-Newtonian, and different considerations are required to measure their viscosity. This is only valid for Newtonian fluids, where at a certain temperature, the viscosity would remain constant as a function of changes in shear rate (e.g., water). The shear stress is the force per area needed to move a laminar layer of fluid, whereas the shear rate is the relation of speed between two laminar layers of fluid. Therefore, viscosity is defined as the internal friction of a fluid, or in other words, its capacity to resist flow.Ĭonsidering a fluid as a number of laminar layers, the formula describing viscosity is straightforward, being represented in centipois (cP) or Pascal seconds (Pa s): When a force or stress is applied to a fluid, it flows, resulting in internal friction that resists the flow.
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