Measuring the rheological properties of different types of fluidic systems is vital for determining the properties of the system under different circumstances, and these properties can tell engineers a lot about the flow characteristics of the fluid. There are many methods for Newtonian fluids, but only a select few methods for non-Newtonian fluids. In this article, we look at a technique that can be used for both types of fluid systems, and that is by using sensors.
The rheological properties of a fluid are important, as they determine how the fluid flows and some of its features. As rheology is an area that is concerned with how liquids flow, the viscosity of a fluid plays a significant role in the rheological properties that a fluid possesses. In many cases, this is what is measured.
While there are many different methods to determine a fluid’s viscosity alone, rheology is bigger than this and can be used to determine the properties of non-Newtonian liquids as well. This extends to include more functional fluid-like systems such as emulsions, paints, solvents, and oils, which are products that have real-world uses.
For many Newtonian-based fluids, the viscosity is the main measurable property and can simply be measured using a viscometer. For non-Newtonian liquids, the standard choice has become different types of sensors that can measure the rheological properties in real-time.
Aside from the viscosity, these sensors can also measure the shear-rate dependent viscosity (i.e., shear-thickening), shear-thinning properties, oscillatory deformation, and the shear modulus of the fluid. In many cases, the properties of the fluid are determined by the fluid passing over a ‘sensing surface,’ so many of the sensors used for studying the rheological properties of a fluid are in-line (or in-phase) sensors. Here, we will look at a few types.
There are various electrochemical sensors found today, some of which use electrical resistance principles and others which apply electromagnetic principles.
The first type, which is a type of electrical resistance rheometry, is a form of in-line (or in-pipe if the fluid is in a pipe) sensor. These types of sensors cross-correlate fluctuations in the fluid and the radial velocity profile and other rheometric data of the fluid is computed using tomography sensors and regions of the pipe which are divided into pixelated regions. Both Newtonian and non-Newtonian fluids can be used with this type of sensor, and they are known to possess a high accuracy for both. It is a technique that can also provide information on the localized and full system mixing behaviors.
The second approach is through the construction of a fluid process viscometer by combining a conventional electromagnetic viscometer with a differential pressure sensor. This is a non-invasive sensor system that enables the velocity profile and viscosity of the fluid to be obtained. The system utilizes an external magnetic field that has a spatially varying magnitude. So, when the fluid flows through the magnetic field, the voltage in the device changes based on the shape of the fluidic flow and this can be used to determine the flow and rheological characteristics of the fluid.
There are quite a few different resonator sensors that can be used, and they have become an alternative option in recent years because they are highly sensitive at low viscosities, can be made very small, and can be used in conjunction with automated and online data systems. Some of the more common resonator sensors are plate resonators and metallic spring-plate resonators.
The first type is the plate resonator, which is highly dependent upon the flow characteristics causing a change in the resonator. Therefore, these devices are more significant than some of the other types of resonator sensor because the dimensions of these devices are large compared to the shear-wave penetration depth of the fluid. Given their size, they are not always the most practical choice.
The second type is the metallic spring-plate resonator, which often consists of a sensitive component suspended on a spring-like structure which is attached to a printed circuit board (PCB). These sensors utilize a permanent magnetic field which causes a current to be applied in-plane to where the liquid flows. Because these sensors employ a high voltage, measurements can be taken between different terminals. As for the measurement itself, when the fluid passes through the magnetic field, it changes the resonance frequency of the magnetic field which is quickly measured and can be used to determine the characteristics of the fluid.
A growing area of fluid rheology sensors is microelectromechanical systems, otherwise known as MEMS. Various types of MEMS-based devices can be used as sensors in the measurement of a fluid’s rheological properties, but we’re going to look at the general mechanism of how these MEMS devices work.
In general, MEMS devices employ a vibrating cantilever - just like those that can be found within AFM machines - and this cantilever is immersed into the fluid being analyzed. When the fluid moves past the cantilever, it exerts a hydrodynamic force on to the cantilever because of shear forces and pressure forces from the fluid. The extent by which the cantilever accelerates under these forces and the subsequent displacement that occurs is used to determine the rheological properties of the fluid.
Sources and Further Reading:
- “Electromagnetic In-line Measurement of Viscosity”- Hans A., IEEE Instrumentation and Measurement¸ 2002, DOI: 10.1109/IMTC.2002.1007095
- “In-pipe rheology and mixing characterization using electrical resistance sensing” Machin T. D. et al., Chemical Engineering Science, 2018, DOI: 10.1016/j.ces.2018.05.017
- “Resonator Sensors for Rheological Properties - Theory and Devices”- Reichel E. K. et al., Proc. AMA Conferences, 2013, DOI: 10.5162/sensor2013/D7.1
- “MEMS-based Measurement of Rheological Fluid Properties”- Dufour I. et al., Proc. AMA Conferences, 2013, DOI 10.5162/sensor2013/D7.4