Though most famous for his theories on gravity, it seems that Newton did work in a lot of less-well-known areas. One of these is the study of the viscosity of fluids. In last week’s post, we discussed why it is often so difficult to control 3-roll reverse coat coil coating processes (click here if you missed it or want to review). In that little dissertation, we spoke very briefly about the non-Newtonian properties of coatings and how they affected that process. Let’s dig a little deeper into the viscosity topic and how Newton, and his work with fluids, affects you every day…
What is “Viscosity”?
Most of us who work with fluids every day understand that viscosity is one of the most important parameters we deal with. In short, viscosity is a definition of a fluid’s resistance to flowing. In laymen’s terms, we often refer to it as a fluid being “thick” or “thin”. This is pretty easy to understand. Water has a low viscosity, and it flows easily (it’s “thin”). Honey and Ketchup have a high viscosity, and they do not flow as easily (they’re “thick”). Heinz created a full blown marketing campaign out of this concept and actually made it their differentiating feature in a commodity market! They made it the standard by which to judge the quality of all Ketchups, and in doing so, set themselves apart as being “the best”!
Enter Temperature – The Great Viscosity Control!
It has long been understood that the viscosity of a fluid is inversely proportional to its temperature. That’s a long-winded way of saying that, as a fluid gets warmer, its viscosity gets lower – it gets “thinner”. We always use honey as the perfect example. When it is cold, you can scoop it. You can’t even get it to pour out of the jar! But heat it up just a little bit and it runs like water – right off your toast!
So, temperature then, is the great viscosity controller. Increase the temperature of the fluid to reduce its viscosity, and cool it down to increase its viscosity.
Enter Newton – And a Whole New Level of Complexity!
As it turns out, everything we talked about to this point applies to virtually all fluids. And this is where Newton comes in…
Newton introduced the concept of shear stress into the viscosity equation. He assumed that viscosity was dependent on temperature, but independent of shear. But when applying his equations, we observe that many fluids, when placed under shear – by squeezing them between two plates (like a lubricant) or by passing them through an orifice (like a coating), for instance – also reduce their viscosity. This is called “shear-thinning”.
Now I always had trouble in school with the “fiction/non-fiction” thing. Non-fiction is true, fiction is made-up. Seems like an unnecessary double-negative to me! Turns out, this Newtonian thing is the same way. Non-Newtonian fluids are affected by shear, whereas Newtonian fluids are not.
Weird. And a little confusing. But let’s just get past that…
Fluids like water, alcohol, motor oil – all are Newtonian. Their viscosity is the same regardless of shear. Virtually all other fluids – and this includes most paints, sealers, adhesives – are Non-Newtonian. Their viscosity varies as a function of shear.
Sources of Shear
There are a lot of places where shear originates. It can be as simple as the flow of the fluid through a pipe, where there is friction between the layers of fluid as they rub against one another, and the wall of the pipe. This friction is dependent on the rate of flow and how turbulent the flow is. As for the piping, each elbow, tee, and pipe fitting adds friction and shear into the system.
Obviously, a pump is a great source of shear – as it compresses the fluid and increases its velocity through the system. And some systems use pressure and flow regulators, both of which introduce a great deal of shear into the fluid. Nozzles and orifices are also great sources of shear. In fact, almost every component in the flow path of modern fluid dispensing systems add shear to the fluid.
Does It Matter?
As it turns out, for industrial fluid users, this may or may not be a major factor in the performance of their system! Let’s take a paint circulation system, for example. The fluid path is “static” meaning that it does not change. The flow rate is also held constant, so turbulence is constant. The pressure in the system is held constant by a backpressure regulator, so the amount of shear it introduces is also relatively constant. As the paint is circulated over-and-over through the system, the shear rate is constant, so it is repeatable, and so, becomes a constant in the system rather than a variable. In this case, the paint, even though it is shear sensitive, will be at a reasonably consistent viscosity – varying mostly by changes in temperature.
Now let’s take that same paint after it leaves the circulation system and goes to the spray robot. Here, the conditions change drastically!
Now, the flow is starting and stopping based on whether the paint is spraying or not. When it is flowing, it is generating shear in the flow path. Whether or not it is flowing is determined by a valve, which introduces shear when it is open, but none when it is not. Then the paint goes through a gun with a host of passages and a nozzle with a small orifice at the end that atomizes the paint as it leaves – the ultimate act of shear!
Careful Evaluation is the Key
In a situation like the paint spray system cited above, the effect of shear on viscosity can be hard to predict and control. It is essential that you evaluate each of the fluid dispensing systems that you are using to determine how shear will affect you. And it is important to note that, as in our paint system example, your system may be a combination of cascaded sub-systems, each with their own set of shear parameters that you have to take into account.
It can be very confusing!