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Viscosity and Beyond: Basics of Applied Rheology
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Viscosity and Beyond: Basics of Applied Rheology By Gina Paroline
Measuring viscosity is a tried and true technique for characterizing uids. But frequently viscosity measurements are not made correctly and accordingly do not provide reliable or meaningful information. Viscosity is a parameter used to describe the ow properties of uids. Measurement takes place by applying a force to the uid and measuring the resulting ow. So, if a sample does not ow, viscosity is not the best parameter to describe its physical properties. Too often it is assumed that vi scosity is a material function which is not impacted by the conditions under which the measurement is made. For some uids, i.e. simple uids, viscosity is a material function dependent only on the temperature and pressure at which the measurement is made. Simple uids have no dependence on the shear conditions under which the viscosity measurement is made. However, many uids of great commercial interest are not simple. Instead they are complex in that they are mixtures of components in different phases (solids, liquids, gases). Such complex uids have structure within them which is sensitive to shear resulting in their viscosity not being a single value at a given temperature and pressure but an array of values depending on the amount of shear applied. Simple uids include materials such as water, solvents, oils, and syrups (at certain temperatures). For these types of materials, the viscosity can be measured under any shear condition and the result will be the same for a given temperature and pressure. But for those very important complex uids, viscosity is a function of shear. To obtain meaningful viscosity data, one must consider what shear conditions are relevant for the task at hand.
For viscosity measurements to be meaningful, they must be conducted at a shear rate appropriate for the technical application requiring the viscosity data.
By varying the applied shear, as either a shear stress or shear rate in a rotational viscometer or rheometer, the sensitivity to shear can be assessed and samples ranked based upon the measured viscosity at the various shear conditions as well as the slope of the curve when these viscosity points at the various shear conditions are connected. For all but simple uids, viscosity is a curve and to obtain meaningful data the viscosity should be measured over as broad a range of shear conditions possible. For instance, if one only measures viscosity at a shear rate of 100 1/s, that measured point may do well at predicting the performance of a sample during pumping but it would likely not provide any useful information about the sample’s sedimentation stability.
::: White Paper Modulus is the stiffness of a material. When a force is applied to a solid material it deforms a certain amount. This relationship between the magnitude of applied force and the magnitude of the resulting deformation is modulus (shown below as shear modulus G).
Solids also have the interesting property of maintaining their shape and have the ability to store the energy from the applied force then use that stored energy to retain their shape once the force is removed. In the example above, these two materials have extremely similar viscosity at a shear rate of 100 1/s but are dramatically different materials as witnessed by their viscosity a lower shear conditions. When shear is applied to structured uids, without doubt the structure is damaged to an extent relative to the amount of shear. The change in viscosity with shear in the example above stems from this change in structure. For complex uids, viscosity for a material should be measured over the full range of shear conditions that material will experience during its life from low shear storage conditions, to medium shear use conditions, to high shear production conditions. But what if the material does not ow? If viscosity is a measure of how much a sample ows under an applied force, then viscosity is not an appropriate parameter to describe materials that do not ow. Now the need for rheological measurements is seen. Rheology is the study of the ow and deformation properties of materials. The viscosity measurements described above fall into the “ow” part of this denition so rheometers are also viscometers in that they can measure the viscosity of uids. Materials with much structure, such as solids, do not ow under an applied force rather they deform as a result of the applied force. For such materials, instead of speaking in terms of viscosity, the correct parameter to describe their physical properties is modulus.
From a rheological measurement, not only the stiffness (modulus) of a solid can be determined but its ability to store energy and retain shape (elasticity) is as well. For this reason, often when reading about rheological measurements the terms solid-like and elastic behavior wil l be used interchangeably. Beyond measuring viscosity, rheometer have the ability to measure modulus by precisely controlling the applied strain (deformation or displacement) while measuring the required shear stress (force or torque for a rotational rheometer) or vice versa. So with a rheometer, all kinds of materials can be characterized from simple liquids to pure solids. But the tricky aspect is that most materials of commercial interest have both uid ow (viscous) characteristics and solid deformation (elastic) characteristics, i.e most materials are “viscoelastic”. The materials shown below between the pure liquid on the left and the pure solid on the right show the vast array of viscoelastic materials.
::: White Paper Quite often the desired performance characteristics of a material are governed by the balance between its viscous and elastic characteristics. Providing this critical information on the viscoelastic balance is where a rheometer becomes indispensable.
The ratio of the magnitude of the i nput oscillation wave and the output oscillation wave is the modulus so the stiffness of the material is known. And the dampening, or lag, of the output wave relative to the input wave holds the information on the viscoelastic balance and is called the phase angle.
Viscoelastic balance is important in a sample like a paint because it needs to ow well (viscous behavior) while at the same time having adequate structure at rest to keep the components from separating (elastic behavior).
A simple liquid has a 90° phase angle. A pure solid has a 0° phase angle. All of the viscoelastic materials have phase angles between 0° and 90°. Measurements conducted in this manner are called oscillatory measurements, or dynamic mechanical analysis, and are what set rheometers apart from viscometers.
For a toothpaste, the viscoelastic balance must be such that the paste can ow out of the tube when squeezed (viscous behavior) but then retain its cylindrical shape (elastic behavior) once on the toothbrush.
Sources Cited: Menard, Kevin P. Dynamic Mechanical Analysis. CRC Press, 2008. Print Mezger, Thomas G. Applied Rheology with Joe Flow on the Rheology Road. Anton Paar GmbH, 2015. Print. Mezger, Thomas G. The Rheology Handbook: For Users of Rotational and Oscillatory Rheometers. Hannover: Vincentz Network, 2006. Print
A rheometer goes about measuring viscoelastic behavior by applying a forced oscillation to the sample and monitoring how the sample dampens the oscillation. An input oscillation wave is put onto the sample and an output oscillation wave is measured as the sample’s response. Two useful pieces of information are gained which allow the viscoelastic balance of the sample to be quantied.
