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Rheology

 

VISCOSITY

Viscosity is a flow property of material. The flow of material often occurs during industrial processes, such as drawing copper wire and rolling sheet steel, or making aspirins in a tablet press, as well as mixing and formulating liquids and pastes, or dipping or spray painting an automobile.

Flow occurs in materials normally classed as fluids, pastes, or semi-solids; such as cosmetics, chemical specialties, foods, and miscellaneous products like plotting compounds, varnishes, impregnating materials, adhesives and the like. Viscosity measurement procedures applicable both to manufacturers of these flowable materials and their users. Measurements are important in predicting the performance of these materials.
To better understand complex flow properties, consider how two common materials flow. To set the numbers in their proper perspective, remember that higher numbers refer to materials that have higher resistance to flow.

Which has the higher viscosity, honey or mayonnaise? Those who say "mayonnaise" may argue that the mayonnaise will not flow out of an inverted pint jar, and the honey will. Those who say "honey" may argue that it is more difficult to stir a jar of honey with a wooden spoon than it is to stir a jar of mayonnaise. This is not a paradox, it is simply evidence that there is more to the discussion and description of viscosity than a single number designation like "SAE-30", "325 SUS", or "26 Zahn seconds" or 100 cps.

So, what is viscosity? Simply stated, it is a material's resistance to flow. To more precisely measure and describe the flow of a material, it is often necessary to confine it, to make flow occur mechanically, to measure the force required to do so, and to convert the measured forces to specific values that can be compared with one another.

Imagine a flat square metal plate, anchored to prevent it from moving, and covered with a thin film of grease. Now imagine another plate the same size as the first, placed on top of the film of grease. To slide the top plate horizontally, a force must be applied to it. A small force is enough to move the plate a small distance at a low speed, while a larger force is necessary to move it at a higher speed. It is more difficult to move the plate with only a thin film of grease between the two than it is when there is a thick layer. A larger plate would be more difficult to move than a smaller one, even at the same thickness and speed. Assigning numerical values to these concepts allows them to be handled mathematically; assigning names to their interactions allows them to be discussed and understood.

SHEAR STRESS
The force required to move the upper plate is related to the area in contact with the substance. To arrive at a specific measure of this force, it is necessary to divide the total force needed for motion by the area of the plate in contact with the flowing substance (the grease). This figure is called "shear stress". (Parallel motion between planes is always referred to as "shear".) The customary unit of force is the dyne (about 1000 dynes equal the weight of a mass of 1 gram), and the customary unit of area is the cm2.

In the S.I. or modern metric system, the unit of force is the newton (a newton equals approximately the weight of a mass of 100 grams), and the unit of area is the square meter or m2. In addition, the S.I. system has assigned the name pascal to the unit newtons/m2.

SHEAR RATE
The shearing done on the material is related to the speed of relative movement and the distance between the plates: at a given speed, more work is done on a given unit of material when the plates are closer together. The specific measure of this work per unit of material is called the shear rate, and is defined as the relative velocity divided by the distance between the plates.

Viscosity itself is defined as the ratio between shear stress and shear rate.

Rheology-Fluid categorization

rheology

shearvsstress

On the above graphs, shear rate is plotted against shear stress and viscosity is plotted against shear rate. This relation of viscosity to shear rate is not necessarily linear. This behavior is called pseudoplastic flow.Honey, however, has a linear relationship: the ratio of shear stress to shear rate is a constant as linear through the origin and as a constant level of viscosity independent of shear rate. This behavior is called Newtonian flow. Another straight-line curve is that of plastic flow, but in contrast to pseudoplastic and Newtonian flow, the curve does not pass through the origin (where small forces produce small movements). Instead a small force produces no movement at all, and nothing moves as the applied force is gradually increased, until a point is reached where the material yields, and flow begins. This is called the yield point. True "plastic" materials have linear flow behavior above the yield point, but some materials with a curved "pseudoplastic" flow curve also have a yield point. The fact that the force of gravity is not enough to overcome the yield point is what keeps the mayonnaise in the inverted jar.
A fourth type of behavior is called dilatant flow, characterized by a comparatively greater resistance to flow at higher shear rates or speeds; that is, its viscosity increases with shear rate. Plastisols and concentrated cornstarch/water slurries (un­cooked) are examples.
One last term should be considered, whose definition is not standardized: thixotropic flow, the meaning of which varies in different industries, and even among individuals.
Thixotropic flow is usually characterized by 1) a yield point, 2) plastic or pseudoplastic behavior, 3) a breakdown or reduction in viscosity on continued shearing, visible over a finite time, and 4) usually a tendency to rebuild viscosity and/or yield point on standing. Thus, for instance, when stirring a thixotropic material, it will take some force to start the motion. Then, after it begins to move, the force needed to sustain motion (even at the same speed) will decrease as the shearing (stirring) continues. Immediately after the stirring stops, the material may or may not have yield strength, but on further standing will develop a yield point and/or higher viscosity than it had immediately before stirring ceased. It may or may not recover 100% of its initial yield point or viscosity.