Tackling rheology

Rheology is defined in the Webster Dictionary as the study of changes in shape and the flow of matter, including elasticity, viscosity and plasticity. We will focus in this chapter on viscosity, defined as "the internal friction of a fluid, caused by molecular attraction, and giving it a tendency to resist flow". Your Brookfield viscometer measures this friction and thus functions as a rheology instrument. The aim of this chapter is to educate you on the different types of flow behavior and to use your Brookfield viscometer as a true rheology instrument that allows you to perform detailed analyzes of virtually any type of fluid. This section is of interest to any viscometer user, especially to those who are followers of theoretical and academic schools of thought regarding viscosity measurements.

Viscosity

Viscosity is the measure of the internal friction of a fluid. This friction occurs when a layer of fluid is set in motion relative to another layer. The greater the friction, the greater the amount of energy required to generate the movement which is referred to herein as shear. Shear occurs when the fluid is physically moved, such as when it is poured, applied, sprayed, mixed, etc. Highly viscous fluids require more energy to move than less viscous fluids

Isaac Newton defines viscosity by considering the diagram shown in Figure 4-1. Two parallel planes of fluid of equal area A are separated by dx and move in the same direction but at different speeds v1 and v2. Newton states that the force required to maintain this difference in speed is proportional to the difference in speed through the liquid, or speed gradient. To express this, Newton writes:

or is a constant characteristic of a given product called its viscosity.

The speed gradient is the measure of the change in speed at which the intermediate layers move opposite each other. It represents the shear that the liquid undergoes and is therefore called the shear rate. We will henceforth represent it by the letter S. Its unit of measurement is the second power –1 (sec-1). The term F / A represents the force required per unit area to produce the shearing action. It is called shear stress and is represented by F '. Its unit of measure is dyne per square centimeter (dynes / cm2), or N / m2 or Pa. Using these simplified terms, viscosity can be defined by the following mathematical formula

The unit of measure for viscosity is poise. A product requiring a force of 1 dyne per square centimeter to produce a shear rate of 1 s-1 has a viscosity of 1 Poise, or 100 centipoise. You will also encounter viscosity measurements expressed in Pascal-seconds (Pa.s) or milliPascal-seconds (mPa.s). These are the units of the international system and are sometimes preferred over the CGS system. One Pascal-second is equivalent to 10 Poises. A milliPascal-second is equal to 1 centiPoise. Newton considers that any material has, at a given temperature, a viscosity which is independent of the shear rate. For him, double the applied force will cause the displacement to be twice as fast. As we will see, Newton was only partially right.

Newtonian fluids

This type of flow behavior which Newton considered applicable to all fluids is unsurprisingly called Newtonian behavior. However, this is only one case among several other flow behaviors that you may encounter. A Newtonian fluid is represented graphically in Figure 4-2. Graph A shows that the relationship between the shear stress (F ') and the shear rate (S) is a straight line. Graph B shows that the viscosity of the fluid remains constant regardless of the shear rate. Typical Newtonian fluids are, for example, water and engine oils.

This means in practice that, for a given temperature, the viscosity of a Newtonian fluid remains constant regardless of the viscometer model, the type of moving body and the speed used to measure it. Brookfield standard oils are Newtonian over the shear range generated by Brookfield equipment. This is why they can be used with all our viscometer models (except with the CAP 1000 and CAP2000 models). Newtonian fluids are arguably the easiest fluids to measure. Just grab your viscometer and measure… They are unfortunately not as common as fluids in the more complex non-Newtonian group.

Non-Newtonian fluids

A non-Newtonian fluid is globally defined as being a fluid whose F '/ S ratio is not constant. In other words, when the shear time varies, the shear stress does not vary in the same proportions (and not necessarily in the same direction). The viscosity of such products therefore changes when the shear varies. Thus, the experimental parameters such as viscometer model, mobile and speed have an effect on the viscosity measurement of a non-Newtonian product. The measured viscosity is then called apparent viscosity and this measurement can only be used if the experimental parameters of the measurement are provided. Non-Newtonian flows can be visualized by thinking of fluids made up of a mixture of molecules of different sizes and shapes. When they pass close to each other, as they do during a flow, their sizes, shapes and cohesive powers will determine how much force will be required to move them. At each specific shear rate the alignments may be different and more or less force will be required to maintain motion. There are several types of flow behavior for non-Newtonian fluids. The most common types that you will come across are:

Rheofluidifier:

This type of fluid exhibits a decrease in viscosity as the shear rate increases, as shown in Figure 4-3. Probably the most common of non-Newtonian fluids, shear thinners include paints, emulsions, and dispersions of all types. This type of flow behavior is sometimes referred to as “shear thinning”.

Rheo-thickener:

The increase in viscosity with increasing shear rate is characteristic of this type of fluid. Although rarer than shear flow, shear thickening behavior is frequently observed in fluids containing a high level of deflocculated solids, such as clays, corn starches mixed in water and water / sand mixture. The dilating behavior is also called "shear thickening".

Pseudoplastic

This type of fluid behaves like a solid when it is at rest. A certain force must be applied to this fluid to make it move. This force is called the “yield point”. Ketchup is a good example of this type of product. Its yield point prevents it from flowing out of the bottle unless the bottle is shaken, which then releases the flow. When the yield point is exceeded, the fluid may exhibit Newtonian, shear-thinning or shear-thickening flow.

Until now, we have only mentioned the influence of shear on the flow behavior of non-Newtonian fluids. What happens when time has to be considered? This question leads us to examine two new types of non-Newtonian flow: thixotropy and anti-thixotropy.

Thixotropy and anti-thixotropy

Some fluids show a change in viscosity over time under constant shear conditions. There are two categories to consider:

Thixotropy:

As shown in the following figure, thixotropic fluids show a decrease in viscosity over time under constant shear.

Anti-Thixotropy

It is the opposite of thixotropic behavior, the viscosity of the fluid increases with time under constant shear.

Thixotropy and anti-thixotropy can exist in combination with all of the other behaviors discussed previously or only at certain shear levels. The time element is extremely variable. Under constant shear, some fluids will reach their final viscosity in a few seconds, while for some others, stabilization may take several days. Anti-thixotropic fluids are rare. Thixotropy is frequently observed in materials such as greases, printing inks and paints. When subjected to varying shear, thixotropic fluid reacts as shown in Figure 4-8. A shear stress / shear rate curve was made by increasing the shear to a certain value and then decreasing it to its starting point. Note that the ascent curve and the descent curve do not coincide. This hysteresis curve is due to the drop in viscosity of the fluid over time. These effects can be reversible or irreversible. Some fluids, if left to stand for a while, will return to their original viscosity, while some will never return.

The rheological behavior of a fluid can obviously have important effects on viscosity measurement techniques. In section 4.7 we will discuss some of these effects and how to deal with them. Chapter 5 presents advanced mathematical techniques used to analyze flow behavior under a wide variety of conditions. However, we will first discuss the effects of laminar and turbulent flow on viscosity measurements.

Laminar and turbulent flow

The very definition of viscosity implies the existence of what is called laminar flow, that is to say the movement of one layer of fluid relative to another without transfer of material from one to the other.

Depending on a number of factors, there is a maximum travel speed where material transfer can occur. This phenomenon is called turbulence. Larger molecules or particles jump from one layer to another and dissipate a substantial amount of energy in the maneuver. The net result is that more energy is required to maintain turbulent flow at the same speed as laminar flow.

The increase in energy required is manifested by an apparently greater shear stress than that observed under laminar flow at the same shear rate. The end result is an erroneous viscosity reading because it is more important.

The transformation of a laminar flow into a turbulent flow also depends on other factors than the speed of movement of the layers. The viscosity of the product, its specific weight, the geometry of the mobile used and the size of the container also influence the onset of the moment at which turbulence appears.

Care should be taken to distinguish between turbulent flow conditions and dilating flow behavior (see section 4.4). In general, the viscosities of shear thickening materials increase steadily as the shear increases. The appearance of turbulent flow is characterized by a relatively sudden and significant increase in viscosity from a certain shear threshold. The flow behavior of these materials above this point can be Newtonian or non-Newtonian.

Due to the relatively low shear rates that most Brookfield viscometers operate at, you are unlikely to encounter turbulent flow unless you measure viscosities less than 15 cP with the LV series viscometers and less than 85 cP with the other models. The higher the viscosity of the fluid, the less likely it is to experience turbulence. If turbulence is observed when measuring low viscosity fluids, it can very often be eliminated by using the ULA accessory.

What affects the rheological properties?

Viscometer data often presents itself as a "window" through which other material properties can be observed. Viscosity is more easily measured than most of the properties that affect it, which makes it an interesting tool for the characterization of materials. Earlier in this chapter, we looked at the different types of rheological behavior and how to identify them. After identifying the particular rheological behavior of your product, you may ask yourself what this information implies regarding its other characteristics. This section, based on information collected over the years by our customers, aims to make you think about the mysteries that your viscometer will help you solve.

Temperature

One of the most obvious factors that can affect the rheological behavior of a material is temperature. Some materials are very sensitive to temperature and a relatively small variation can cause a significant change in viscosity. Other products are not very sensitive to temperature. Taking into consideration the effect of temperature on viscosity is essential in the evaluation of materials which will be subject to temperature variations during manufacture or during end use (engine oils, grease, hot-melt adhesives, etc.)

Shear rate

Concretely, non-Newtonian products tend to be the rule rather than the exception. An appreciation of the effects of the shear rate is thus made necessary for anyone involved in the practical applications of rheological data. For example, it would be disastrous to try to pump a shear thickener through a system, just to see it "solidify" inside the pump, interrupting the total production process. Although this is an extreme example, it should not be underestimated the importance that the shear rate can have. When the product is subject to different shear rates during manufacture or during end use, it is essential to know its viscosity at these shears. If this shear is not known, an estimate should be made. The viscosity measurements must then be carried out at shears as close as possible to the estimated values. Very often it is not possible to measure the viscosity of a product at end use shear rates because these values fall outside the shear ranges of the viscometer. It is then necessary to carry out measurements at different shears and extrapolate the measurements to the desired shear. This is not the most accurate method of obtaining this information, but it is often the only alternative available, especially when the shear rates are very high. In fact, it is always advisable to make viscosity measurements at different shear rates to detect rheological behavior which could have an effect on the manufacturing process or the end use method. When the shear values are unknown or if they are not important, a simple curve viscosity versus rotational speed is often sufficient. Examples of materials which are subject to and which are affected by large variations in shear during manufacture or end use are: paints, cosmetics, liquid latex, coatings, certain food products, and blood in the human circulatory system. . The following table shows typical examples of various shear rates.

Measurement conditions

The condition of the product can have a considerable effect during the measurement of its viscosity. It is therefore important to be aware of this and to have maximum control over the environment of any product you need to measure. First, the viscosity measurement techniques outlined in section 3.3 should be applied. Different variables such as: viscometer model, moving / speed combination, vessel size, presence or absence of caliper, product temperature, sample preparation technique, etc., can affect not only the reproducibility of the measurement but also the real viscosity value of the product you are measuring. Second, other less obvious parameters that can influence the viscosity must be considered. For example, the product may be sensitive to the ambient atmosphere, as is the case with dental products, metallurgical products and by-products from blast furnaces, blood and mucus. In these cases, a favorable controlled atmosphere may be required (see information on purge systems given in section 2.1.10). Another factor that can influence the viscosity measurement is the homogeneity of the sample. It is very often preferable to have a homogeneous sample. Thus, more reproducible results can be obtained. Sometimes, however, a material's tendency to separate into different non-homogeneous layers is its most interesting feature. In this specific case, care should be taken not to agitate, shake or mix the product before studying it.

Time

The time elapsed while a shear stress is applied obviously affects thixotropic and anti-thixotropic materials (time dependent materials). But changes in viscosity in many materials can occur over time even if the product is not subjected to shear stresses. The phenomenon of aging should be considered when selecting and preparing samples for viscosity measurement. Also consider the fact that most materials will undergo viscosity changes during the course of a chemical reaction, and that the viscosity measurement made at a particular time during the reaction can vary very significantly from that measured. at another time.

Pressure

Changes in pressure can cause bubbles to form in dissolved gases, change in volume and distribution of gases, and in some cases can create turbulence. Pressure is not taken into account as often as other parameters. The pressure compresses the fluids and thus increases the inter molecular resistance. Liquids are compressible under the influence of very high pressures in the same way as gases but to a lesser extent. The increase in pressure causes an increase in viscosity. For example, the flow properties of highly loaded sludge (greater than 70-80% by volume of particles), when the liquid phase is insufficient to fill all the holes between the particles, are the result of a mixture of three phases (solid, liquid and air). Due to the presence of air, the mixture is compressible and therefore the more compressed it is, the more it resists flow.

Product history

What happened to a sample before its viscosity was measured can significantly affect the result, especially for fluids sensitive to heat and aging. Thus, storage conditions and sample preparation techniques should be designed to minimize their effects on the viscosity measurement. Thixotropic materials in particular are sensitive to their history because their viscosity will be affected if they have been previously stirred, mixed, poured, or if they have undergone any other action producing shearing. 4.7.7 Composition and additives The composition of a material is one of the determining factors of its viscosity. When its composition is modified, either by changing the proportions of its different components, or by adding another material, a change in viscosity is highly predictable. For example, adding solvent to a printing ink reduces the viscosity of the ink. Additives of all types are used to control the rheological properties of paints.

Specific characteristics of dispersions and emulsions

Dispersions and emulsions are multiphase materials consisting of one or more solid phases dispersed in a liquid phase. They can be rheologically affected by a number of factors. In addition to the majority of the factors described above, the strange properties of multiphase materials have a very significant part in the rheology of such materials.

One of the main characteristics to study is the state of cohesion of the material. Are the particles that make up the solid phase separate and distinct or are they grouped together? How big are the groups and are they strongly united? If the groups occupy a large volume in the dispersion, the viscosity will tend to be higher. This is due to the greater force required to break the bonds of cohesion of the suspended solid matter.

When the blocks are agglomerated in the dispersion, the behavior of the agglomerate can induce shear thinning flow. At low shear, the agglomerate can deform but remain relatively intact. When the shear increases, the agglomerate can break down into several individual blocks, decreasing the areas of friction, therefore reducing the viscosity. See section 4.4 for more details on shear thinning behavior. If the bonds in the agglomerate are very strong, the system may have a yield point (see section 4.4 on plastic behavior). The importance of the threshold value depends on the force required to break these bonds.

If the flocculation structure of the material is destroyed over time while it is subjected to a fixed stress, a flow behavior of the “time dependent” type will be observed (see section 4.5).

If the shear rate is reduced after destruction of all or part of the flocculation structure, the viscosity of the material may be lower than what it could have been in the past at the same shear rate. As the blocks begin to come together again after destruction, the speed at which this gathering occurs affects the time to reconversion back to the initial level of viscosity. If the speed is fast, the viscosity will be quite similar to the initial value after a short time. If the speed is slow, the viscosity will stay at a lower level for a longer time. This is characteristic of a rheological behavior called thixotropy (see section 4.5).

The particle-to-particle attraction of the dispersed solid phase depends on the type of material present at the interface between the liquid phase and the solid phase, which in turn affects the rheological properties of the system. Thus, the introduction of flocculants or anti-flocculants into the system is a method for controlling its rheology. The shape of the particles constituting the dispersed phase is also important when determining the rheology of the system. Particles suspended in a fluid medium are constantly rotating. If these particles are essentially spherical, the rotation can be free. If, however, the particles are sharp or flattened in shape, the ease of rotation is less predictable, as is the effect of varying the shear rate. The stability of a dispersed phase is particularly critical when measuring the viscosity of a multiphase system. If the dispersed phase tends to settle, then producing an inhomogeneous fluid, the rheological characteristics of the system will change. In most cases, this means that the measured viscosity will decrease. Data obtained under these conditions will often be erroneous, requiring special precautions to ensure that the dispersed phase remains in suspension.