Non-Newtonian behavior can be observed in a significant portion, if not the vast majority, of fluids. When the shear rate at which a fluid's measured viscosity is taken depends on the shear rate at which the measurement is taken, we call that fluid a non-Newtonian fluid.
When a fluid is subjected to uniform shearing, the ratio of the shear stress to the shear rate is used to define the viscosity of the fluid.
- Newtonian describes the state of the fluid in which the drag force, also known as the shear stress, is proportional to the velocity of the lower plate, also known as the shear rate
- Its thickness is proportional to the ratio of the force of friction to the speed at which it is moving
- Because of this, the viscosity is not affected by the rate at which it is being sheared; rather, the shear stress will simply increase to match the growing rate of shearing
- The ratio of shear stress to shear rate is one of the most important factors in determining viscosity
If you apply a different force to a Newtonian fluid, the viscosity of the fluid will not change regardless of how much force you apply. Regardless of the amount of force that is exerted, there is no change in the viscosity. The relationship between shear stress and shear rate is linear in a Newtonian fluid, and it passes through the origin. The coefficient of viscosity serves as the constant of proportionality in this relationship.
The amount of force that is exerted on a non-Newtonian fluid causes its viscosity to shift, causing the fluid to behave differently depending on the amount of force. The viscosity shifts whenever there is a change in the amount of force that is applied. It is not possible to define a constant coefficient of digital viscosity meter for non-Newtonian fluids in the same way that this is possible for Newtonian fluids. This is because the relation between the shear stress and the shear rate in a non-Newtonian fluid is different, and it is even possible for this relation to be time dependent (Time Dependent Viscosity).
In the case of non-Newtonian fluids, the concept of digital viscosity meter that is typically utilized in fluid mechanics to characterize the shear properties of a fluid is not quite adequate. This is because non-Newtonian fluids do not follow the Newtonian model of fluid behavior. Instead, they are studied most effectively through a number of other rheological properties that relate to stress and strain rate tensors under a wide variety of flow conditions. These flow conditions are measured utilizing a variety of rheometers and devices.
The decrease in measured viscosity that occurs when the shear rate of the viscometer is increased is an example of a type of non-Newtonian behavior that is exhibited by a large number of fluids. This type of behavior is known as shear thinning. The ratio of shear stress to shear rate is initially high in a shear thinning liquid, but this ratio begins to decrease as the shear rate increases. The shear stress versus shear rate curve for the shear thinning liquid can be seen above in Fig. 2. This curve begins parallel to the one for the high viscosity fluid and ends up parallel to the curve for the low viscosity fluid. The more quickly the fluid is sheared, the thinner it will become.
The relationship between shear stress and shear rate is typically non-linear for the vast majority of fluids. This indicates that the viscosity that was measured varies depending on the kind of viscometer that was applied to make the measurement. The shear rate of the SRV is significantly greater than that of the majority of rotational, capillary, and efflux cup viscometers. Because of this, the SRV will frequently show a significantly different viscosity than a laboratory rotational instrument will.
Measurements of a typical shear-thinning fluid are going to be discussed in the following paragraphs, along with the implications those measurements have for inline measurements using the Rheonics SRV. Even though the shear rate is not well defined for the majority of viscometers, it has been demonstrated that the shear rate of vibrational viscometers, such as the Rheonics SRV, is over one hundred times higher than that of a typical Brookfield, Fann, or other rotational viscometer. This is the case even though the shear rate is not well defined for the majority of viscometers. This indicates that the SRV operates on the high-shear tail of the shear rate curve. Case in point:When compared to the majority of non-vibrational methods used to measure viscosity, it provides significantly lower readings for the viscosity being measured.
The fabric softener has a digital viscometer of 9.7 cPs when measured with a Rheonics SRV viscometer, which indicates that its thickness is fairly thin. The phenomenon depicted in Figure 1—namely, the reduction in digital viscometer (add it to your favorites) that occurs in response to an increase in spindle speed—is responsible for the significantly lower value. The shear rate of the SRV is approximately two orders of magnitude higher than what is attainable with typical rotational viscometers, which indicates that the material is shear-thinning. Because of this, it is not possible to get a numerical match between the SRV and rotational viscometers other than in the case of strictly Newtonian fluids, which are fluids that are not affected by the shear rate.
The monitoring of the of a product in order to confirm that the product's has not changed over time is a common application for an inline viscometer. The operator may then make adjustments to the process parameters in order to maintain the viscosity within a predetermined range. In that case, it might be possible to derive a conversion formula that enables Rheonics SRV readings to be correlated with laboratory measurements under defined shear conditions. This would be possible if the previous sentence is true.
Alternately, the Rheonics SRV could be used as the sensor in a control loop to enable automatic adjustment of one or more process parameters. This would ensure that the viscosity of the product remained within the parameters that had been set. One common use for the Rheonics SRV is in printing machines that use flexographic or rotogravure printing; in these machines, the SRV is used to maintain the same level of in the printing ink.
When highly non-Newtonian fluids, such as products that contain soap and surfactant, are being monitored or controlled, the SRV has to be operated in a fluid that is flowing past the sensor. This is the case regardless of which scenario is being considered. If it is simply submerged in a beaker of product, it will not provide accurate readings that can be replicated. It will give a clear and reproducible response to any changes in the viscosity of the product when it is used in a line that contains flowing product and will give that response.