Bringing Theory into Focus

Published: April 05, 2013, By Mark Miller

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Coating specialists love to throw around big ideas and even bigger words (rheology, viscosity, surface tension, etc.). But what is important? What can you use daily as an applications individual in the fluid coating world?

So let’s start with the most important concept to understand in coating – viscosity. Viscosity is fundamentally the internal resistance to flow that a fluid has. There are fluids that are similar to water (Newtonian) and fluids that are polymers (non-Newtonian). Polymers are either shear thickening or shear thinning. Shear thinning fluids are more common and the easiest way to imagine a shear thinning fluid is to consider mayonnaise. Mayo thins as you spread it. In other words, as you apply shear to the mayo, it is easier to spread. Shear thickening fluids are less common and the easiest way to imagine a shear thickening fluid is to consider honey. Honey becomes more difficult to spread with shear force.

Even with Newtonian fluids, it is helpful to picture spreading the fluid over a stable piece of bread (the substrate in this example) with a moving knife (the coating applicator). The knife acts as the moving force that the fluid is resisting. The more the resistance to flow, the higher the viscosity.

Viscosity, however, is not the only force acting on a fluid in a coating application. Surface energy can play just as big a role as viscosity in the process-ability of a fluid. When a fluid replaces air to coat a solid substrate, there are some surface energy forces that need to be considered. First, how much the fluid likes the substrate versus how much it likes itself is important in the wettability of the fluid coating. This surface energy force is considered surface tension. In addition, there is a surface energy force associated with the curvature present when the fluid is coated onto the substrate, which is referred to as capillary pressure. The surface tension and capillary pressure act in concert with viscosity to explain the fluid behavior in coating flow processes.

The interaction of the viscous force and the surface energy is referred to academically as the capillary number. This dimensionless number is a measure of the ratio of these two critical forces in a coating application. Viscosity and surface energy are material properties that are inherent in the fluids and substrates chosen for a specific product application. However, process controls can fine tune the ability to coat and spread a fluid onto a substrate effectively. Changing pressure gradients at flow exits and contact points have strong effects on the coating system. The introduction of energy systems to tighten or relax surface tension can allow the fluid to flow or stand on a substrate; the reduction of air barriers at the application point of a coating head with vacuum systems can encourage adhesion and wettability; and compression and decompression chambers in coating heads can reduce or induce elastic stresses in the fluid for improved flow and stress-relaxation behavior. Ultimately, the more you can visualize the forces involved in fluid coating phenomena, the more fluent you will be in understanding and interpreting coating applications, products, and processes.

If you are interested in discussing this concept further, contact Mark D. Miller, Founder and CEO of Coating Tech Service, LLC (www.coatingtechservce.com) at This email address is being protected from spambots. You need JavaScript enabled to view it. or (612) 605-6019.