DEFINING VISCOSITY
A fluid’s resistance to flow is its viscosity—the ratio of force required to overcome internal friction between layers of fluid (shearing stress) to the change in speed between layers of fluid (velocity gradient). Viscosity is also described as the internal resistance of a fluid to flow and may be considered as a measure of fluid friction. Data on a material's viscosity also gives manufacturers most valuable knowledge of its characteristics to help predict pumpability and pourability, performance in specific operations, and ease of handling. That’s why it serves an important purpose in the parameter of many industries.
IMPORTANCE OF VISCOSITY
A liquid’s viscosity is an important parameter since it can be used as an indicator of quality. In some instances, a thicker liquid being thought of as superior quality when compared to a thinner product. As it is proven that viscosity measurements are often the quickest, most accurate and most reliable way to analyze the most important factors, it is confirmed to affect product performance.
In different industries, measuring viscosity involves varied techniques and instruments, each dependent to specific products and processes. Selecting the best viscometer is one difficult task since this device varies from the simple to the complex. It is important that one has the “know-how and experience”.
UNDERSTANDING RHEOLOGICAL MEASUREMENTS
The experimental characterization of a material's rheological behavior is known as rheology which is frequently used synonymously with rheometry, particularly by experimentalists. Theoretical aspects of rheology are the relation of the flow/deformation behavior of the material and its internal structure (e.g., the orientation and elongation of polymer molecules), and the flow/deformation behavior of materials that cannot be described by classical fluid mechanics or elasticity.
Since rheology is taken as one of the most effective methods for material characterization, it is frequently found in the area of quality control in which raw materials must be consistent from one batch to another. For this purpose, the indirect measure of product consistency and quality is flow behavior. Another cited importance of flow behavior studies is in obtaining a direct assessment of processability. Just as how a high viscosity liquid requires a higher amount of force to pump than a low viscosity one, this relationship is useful in polymer synthesis, because it allows relative differences to be seen without making molecular weight measurements. Rheological measurements also play an important role in the course of the chemical reaction. This kind of measurements can be employed as quality check process during production or while monitoring and/or controlling an operation. Rheological measurements are also useful in following the course of a chemical reaction. Such measurements can be employed as a quality check during production or to monitor and/or control a process. Rheological measurements are ideal for application in quality control, as they allow to draw conclusions regarding both molecular structure and processability from their results. Using several examples from industrial practice, the capacity of rheological measurements to clarify quality problems like degradation effects or non-homogeneity is demonstrated.
Rheological measurements can detect even the most discrete of changes in structural properties, and are therefore fast & accurate methods of providing quality control for all aspects of the process. Whether it be screening incoming raw materials to discriminate batch to batch variance, avoiding extrusion problems such as melt fracture by mapping the critical shear stresses at which the phenomena occur, or determining thermal degradation in failed components caused by inadequate drying of material / excessive thermal history within the process, rheological measurements can provide this information in a fast & reproducible manner.
Now, "Is it possible that rheological parameter can be employed to correlate with an aspect of the product or process?" Before that question is broached, the anticipated types of chemical and physical phenomena affect the rheological response must be identified first. Then, gather the necessary preliminary rheological data to determine the flow behavior characteristic of the process under evaluation. This basically involves making measurements with whichever Viscometer is available and drawing some conclusions based on the descriptions of flow behavior that follow.
A deeper sense of understanding about the way components of the system interact is established once the type of flow behavior is identified. The obtained results may then be considered as “fit” to one of the mathematical models which have been successfully collated with the help of Brookfield instruments.
These mathematical models range from the simple to the very complex of which some basically involves plotting of data on a graph paper while others require calculations. Others may require the use of programmable calculators or even computers. This kind of analysis is the most effective way of getting the most from the gathered data and often has constant results which can be related to the product or process performance. Once a correlation has been established between rheological data and product behavior, procedure reversal becomes plausible and rheological data may then be used to predict process and performance.
A BROADER APPROACH TO RHEOLOGY
Webster's Dictionary defined rheology as "the study of the change in form and the flow of matter, embracing elasticity, viscosity, and plasticity. Rheological properties can be measured from bulk sample deformation using a mechanical rheometer, or on a micro-scale by using a microcapillary viscometer or an optical technique such as Microrheology. The measurement of rheological properties is applicable to all materials – from fluids such as dilute solutions of polymers and surfactants through to concentrated protein formulations to semi-solids such as pastes and creams, to molten or solid polymers as well as asphalt.
In this chapter we will focus more on viscosity and further define rheology, the internal friction of a fluid, molecular attraction and its tendency to resist a flow. Brookfield Viscometer functions as a tool of rheology as it measures this type of friction. This chapter’s purpose is to help with familiarization of the Brookfield Viscometer as a rheological instrument and the different types of flow behavior. This instrument enables you to conduct a detailed analysis of (virtually) any fluid. This should be of great use to all Viscometer users, particularly those adhering to the Theoretical and Academic schools of thought on viscosity measurement.
VISCOSITY
Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. All real fluids (except superfluids) have some resistance to stress and therefore are viscous, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid. The greater the friction, the greater the amount of force required to cause this movement, which is called shear. Shearing occurs whenever the fluid is physically moved or distributed, as in pouring, spreading, spraying, mixing, etc. Highly viscous fluids, therefore, require more force to move than less viscous materials.
Isaac Newton defined viscosity by considering the model represented in the figure above.
Explanation: Two parallel planes of fluid of equal area A are separated by a distance dx and are moving in the same direction at different velocities V1 and V2. Newton assumed that the force required to maintain this difference in speed was proportional to the difference in speed through the liquid or the velocity gradient.
To express this, Newton wrote: F/A = n (dv/dx)
The difference in velocity between adjacent layers of the fluid is known as a velocity gradient and is given by v/x, where v is the velocity difference and x is the distance between the layers. Velocity Gradient is also defined as the variation in velocity among the adjacent layers of the fluid. The difference in the flow of velocity between the adjacent layers of the fluid is measured in velocity gradient. It describes the shearing liquid experiences and is thus called shear rate. This will be symbolized as S in subsequent discussions. Its unit of measure is called the reciprocal second (sec-1).
The term F/A indicates the force per unit area required to produce the shearing action which is referred to as shear stress – S.
Its unit of measurement is dynes per square centimeter (dynes/cm2).
So viscosity, on simple terms, can be defined as:
Poise is the fundamental unit of viscosity. A material requiring a shear stress of one dyne per square centimeter to produce a shear rate of one reciprocal second has a viscosity of one poise, or 100 centipoise. Whenever you encounter viscosity measurements expressed in Pascal-seconds (Pa·s) or milli-Pascal-seconds (mPa·s) remember that these are units of the International System and can be sometimes used in preference to the Metric designations. One Pascal-second is equal to ten poise; one milli-Pascal-second is equal to one centipoise.
Newton’s viscosity law states that, the shear stress between adjacent fluid layers is proportional to the velocity gradients between the two layers.
The ratio of shear stress to shear rate is a constant, for a given temperature and pressure, and is defined as the viscosity or coefficient of viscosity.
NEWTONIAN FLUIDS
A fluid, whose viscosity does not change with the rate of deformation or shear stain (V/Y), is called Newtonian fluid. A fluid which obeys Newton’s law of viscosity is termed as Newtonian fluid.
Explanation: The graph shows that at a given temperature the viscosity of a Newtonian fluid will remain constant regardless of which Viscometer model, speed or spindle is used for measurement. Brookfield Viscosity Standards are Newtonian within the range of shear rates generated by Brookfield equipment; that's why they are usable with all our Viscometer models. Newtonians are considered the easiest fluids to measure. However, they are not as common as that of more complex group of fluids, which are the non-Newtonians.
NON-NEWTONIAN FLUIDS
A non-Newtonian fluid is broadly defined as the type of liquid whose viscosity changes with the rate of deformation or shear stain (V/Y) and does not obey Newton’s law of viscosity. The viscosity will, therefore, change as the shear rate is varied. This measured viscosity is called the apparent viscosity of the fluid and is accurate only when explicit experimental parameters are furnished and adhered to.
Non-Newtonian fluids have viscosities that change according to the amount of force that is applied to the fluid. The viscosity changes as the force applied changes. At each specific rate of shear, the alignment may be different and more or less force may be required to maintain motion.
In case of non-Newtonian flow behavior, it is differentiated based on the way a fluid's viscosity changes in response to fluctuation in shear rate. The most common types of non-Newtonian fluids include:
Pseudoplastic
This type of fluid is characterized by its rate of flow (as of solutions of rubber or gelatinous substances) increases faster than normal in relation to the shearing stress. This type of fluid will display a decreasing viscosity with an increasing shear rate, as shown in the figure below.
The most common examples of the non-Newtonian fluids, pseudo-plastics include emulsions, paints, and many types of dispersion. This type of flow behavior is sometimes called shear-thinning.
Dilatant
Increase in viscosity and setting to a solid as a result of deformation by expansion, pressure, or agitation; see the figure below. Although uncommon versus pseudoplasticity, dilatancy is frequently observed in fluids containing deflocculated solids of high-level, such as corn starch in water, and sand/water mixtures. This is also referred to as shear-thickening flow behavior.
Plastic
A type of fluid which behaves as solid under static conditions. Flow is induced only after a certain amount of force is applied to the fluid; this force is called the yield value. Cite tomato catsup as a perfect example; its yield value will often make it refuse to pour from the bottle unless the bottle is shaken or struck, allowing the catsup to spurt freely. Once the yield value has been exceeded and flow begins, plastic fluids may then display Newtonian, pseudoplastic, or dilatant flow characteristics. Please see the figure below.
After discussing the effect of shear rate on non-Newtonian fluids, we’ll proceed to the question: “What happens when the element of time is considered?” This question will then lead us to the examination of two more types of non-Newtonian flow: thixotropic and rheopectic.
THIXOTROPY AND RHEOPEXY
There are types of fluid that display a change in viscosity when under time constraints of constant shear rate. There are two categories:
Thixotropy
This is a reversible behavior of certain gels that liquefy when they are shaken, stirred, or otherwise disturbed and reset after being allowed to stand.
Rheopexy
This is, on the other hand, the opposite of thixotropic behavior, the rheological phenomenon in which certain fluids solidify more quickly when subjected to shear.
The time element is extremely variable; under conditions of constant shear, some fluids will reach their final viscosity value in a few seconds, while others may take up to several days. Both may occur in combination with any of the previously discussed flow behaviors, or only at certain shear rates.
Rheopectic fluids, such as some lubricants, thicken or solidify when shaken.
Some liquids behave differently with stress (application of force) over time. Rheopectic liquids increase in viscosity as stress over time increases. Thixotropic liquids decrease in viscosity as stress over time increases.
When subjected to varying rates of shear, a thixotropic fluid will react as illustrated in the figure below. A plot of shear stress versus shear rate was made as the shear rate was increased to a certain value, then immediately decreased to the starting point. Note that the up and down curves do not coincide. This hysteresis loop is caused by the decrease in the fluid's viscosity with increasing the time of shearing. Such effects may or may not be reversible; some thixotropic fluids, if allowed to stand undisturbed for a while, will regain their initial viscosity, while others never will.
LAMINAR AND TURBULENT FLOW
A flow can be Laminar, Turbulent or Transitional in nature.
The flow of a fluid when each particle follows a smooth path, paths which never interfere with one another is the called laminar flow: One result of laminar flow is that the velocity of the fluid is constant at any point in the fluid. Laminar flow is also described as the smooth, orderly movement of a fluid, in which there is no turbulence, and any given sub-current moves more or less in parallel with any other nearby sub-current. This flow type is common in viscous fluids, especially those moving at low velocities.
Considering a number of factors, beyond which an actual transfer of mass occurs, there is a certain maximum speed at which one layer of fluid can move in relation to another and it’s called turbulence. Turbulent flow is a chaotic form of fluid transport in which velocity components randomly fluctuate. This is displayed when molecules or larger particles jump from one layer to another and dissipate a substantial amount of energy in the process. This includes a rapid variation of pressure and flow velocity in space and time. In contrast to laminar flow, the fluid no longer travels in layers and mixing across the tube is highly efficient. The net result is that a larger energy input is required to maintain this turbulent flow than a laminar flow at the same velocity.
Aside from the velocity at which the layers move, the point at which laminar flow transforms into turbulent flow depends on other factors. A material's viscosity and specific gravity together with geometrical dimensions of the sample container and Viscometer spindle, they all contribute to this kind of transition.
It is uncommon for someone to encounter turbulent flow unless when measuring viscosities lower than 15 cP with an LV series Viscometer or 85 cP with other models. Fluids with a higher viscosity of are less likely it is to experience turbulence. In any event of turbulence, while measuring low viscosity fluids, it is often eliminated by using the UL Adapter™ accessory.
In general, dilatant materials will show a consistency in increasing its viscosity with increasing shear rate; turbulent flow is characterized by a relatively sudden and substantial increase in viscosity above a certain shear rate.
FACTORS AFFECTING RHEOLOGICAL PROPERTIES
Viscosity data often functions as a "window" through which other characteristics of a material may be observed. Viscosity is more easily measured than some of the properties that affect it, making it a valuable tool for material characterization. Earlier in this p we discussed various types of rheological behavior and how to identify them. Having identified a particular rheological behavior in a material, you may wonder what this information implies about its other characteristics. This section, based on information gleaned from years of customer experience, is intended as a "tickler" to get you thinking about the mysteries your Viscometer can help you solve.
Temperature
With changes in temperature, there is typically a corresponding effect on the rheological behavior of a material. Temperature is among the obvious factors that impact viscosity. There are materials that are sensitive to temperature that a minimal variation may result in a significant change in viscosity. In the evaluation of materials, it is important to consider the effect of temperature variations in the processing.
Shear Rate
Shear viscosity is the ratio between the pressure exerted on the surface of a fluid, in the lateral or horizontal direction and the change in velocity. Fluid flow is highly dependent on the viscosity of fluids. At the same time for a non-Newtonian fluid, the viscosity is determined by the flow characteristics. It would, for example, be disastrous to try to pump a dilatant fluid through a system, only to have it go solid inside the pump, bringing the whole process to an abrupt halt. While this is may be an extreme example, the importance of shear rate effects should not be underestimated.
Viscosity measurements should then be made at shear rates as close as possible to the estimated values. It is critical to know its viscosity when a material is to be subjected to a variety of shear rates in processing or use. If unknown, an estimate should be made.
For Newtonian liquids in which the molecules interact in thermodynamic equilibrium, the viscosity is an intrinsic material parameter and is independent of the shear rate. Suspending small particles in a Newtonian liquid can bring about non-Newtonian behavior. In this case, viscosity may vary with shear rate, and in the field of rheology, where the term viscosity is used more broadly, the viscosity is usually given as a function of shear rate. However, the extension of the term viscosity to non-Newtonian fluids comes at the cost of some of the rate range of the Viscometer. In this case, it is necessary to make measurements at several shear rates generality of the Newtonian viscosity.
Although not considered as the most accurate method for acquiring this information, it is often the only alternative available when the projected shear rates are very high. It is, in fact, always advisable to make viscosity measurements at different shear rates to detect rheological behavior that may have an effect on a process or application. Where shear rate values are unknown or not important, a sample plot of viscosity versus RPM will often suffice.
There are several examples of materials that subjected to/are affected by wide variations in shear rate during processing and applications of which includes: coatings, paints, cosmetics, liquid latex, certain food products, and even blood in the human circulatory system. The succeeding tables will provide examples of shear rates in variation.
