In Part I of this series, we examined some of the issues with the viscosity cup measurement process and started to look at the fundamentals of fluid viscosity as a means of explaining the source of those problems. In this installment, we pick up the discussion with a somewhat difficult topic: thixotropy.
It turns out that, in addition to Kinematic and Dynamic viscosity, fluids are divided into two additional categories to describe their viscous behavior: Newtonian and Non-Newtonian. Obviously, this is a nod to Sir Isaac, but the difference in these designations describes the definition of thixotropy – so let’s start there…
Newtonian fluids are those who’s viscosity doesn’t vary with shear. While water is most often cited, another good example is motor oil, which you want to maintain its viscosity when under shear stress (for instance, between two metal parts like a piston ring and the cylinder in an engine) to provide lubrication to keep the parts from wearing each other out.
Non-Newtonian fluids, on the other hand, change viscosity as a function of the shear placed on them. This accounts for just about everything else! Virtually all coatings, sealers, adhesives, and other complex formulated fluids used in modern industrial processes demonstrate “shear sensitivity”.
What is Shear?
Shear occurs in a fluid when a force is placed on it. That force may be the pull of gravity (as in our water and honey example in Part I of this series), or it may be from the piston, blade or impeller of a pump as we force the fluid through a pipe or hose. Shear is also created at the wall of the pipe or hose as the fluid at the outside clings to the inner surface and the rest of the fluid flows past it on the inside. The fluid is also “placed in shear” when it is passed through an orifice, such as in a pressure regulator, or the nozzle of an applicator, like a gun or a bell – or the orifice in the bottom of a Zahn cup!
For a thixotropic, shear-thinning fluid, as the stress or shear in the fluid increases, its viscosity decreases.
Author’s Note: There are a few materials that actually get thicker when placed under stress – these are called Dilatant (shear-thickening) fluids. But they are pretty rare. Probably the most common is the mixture of simple cornstarch and water – often called “Oobleck” (from the popular Dr. Seuss story “Bartholomew and the Oobleck.”). Google it to learn more. There are some really fun videos out there!
The whole concept is probably easier to understand with a visual example. Figure 1 shows the characteristic of a Newtonian fluid. As we can see, even though the shear is increasing, the viscosity of the fluid remains unchanged.
The behaviour of Non-Newtonian fluids, however, is a little more complex. As shown in Figure 2, the shear curve (shown in red in the top graph) increases in a non-linear fashion. It increases faster at the beginning and levels out at the end. This is pretty typical of industrial fluid dispensing processes. But what is really interesting is the behavior of the fluid as the shear increases as shown by the green curve in the bottom graph.
For convenience, this is broken into zones to make it easier to understand. Zone 1 (the First Newtonian Range) shows the viscosity when the shear level is low. Under these conditions, the viscosity remains stable. But as the shear increases, it begins to affect the thixotropic material and we see the viscosity falling sharply in response to the shear through the transition range. This is an inversely proportional response, but it has its limits. There comes a point, marked by the entry into Zone 2 (the Second Newtonian Range) where increasing the shear no longer has an effect on the viscosity of the fluid.
The problem comes when you try to operate your process in the transition area between the two Newtonian ranges. Here, small changes in shear, produce fairly large changes in viscosity, which can significantly impact the outcome of your process.
Then again, this can be used to your advantage…
Suppose your material is a coating, sprayed through an orifice onto a part. When it is sprayed, the viscosity is reduced by the shear created by the orifice. When it hits the part, the low viscosity allows the fluid to readily “flow out”. But at this point there is no more shear on the fluid. As a result, it climbs up the curve backward through the transition range, arriving at the higher viscosity of the First Newtonian Range at which point the flow out virtually stops. This is referred to as “recovery”. It is the fluid returning to its “non-sheared” state. How long it takes for that process is called “Recovery Time” and can be measure in seconds, minutes, or in some cases, even hours.
So, how does all of this relate to cup measurements?
We’ll pick up that discussion in the final installment of this series…
1 – Graphs courtesy of Sofraser