Liquids can be described as mayonnaise or beer. The Newtonian fluids are beer – they are self-leveling and if you turn a glass of beer upside down, the beer spills out. The non-Newtonian fluids are mayonnaise – without shear they will not move. Turn the mayonnaise jar upside down and it goes no where. But what happens when you have a wheat beer, or a garlic aioli? The dispersed particles can cause coating defects you weren’t expecting.
Depending on the industry, particles in fluid coating can be common or unusual, but it is good to consider them if you coat fluids for a living, because the particles can add functionality and premium products. In the battery industry, cathodes and anodes are highly filled slurries, where particles are common place. This is also true with microspheres in adhesive coating and nanowires in ITO replacement film. The reality is particles are getting smaller and more important for functionality of active fluid coatings.
While non-functional particles in coating can create defects and are typically filtered out, functional particles need to be included in the coated product and special care needs to be taken to make sure the equipment is designed properly and the process is set up for success. There are a couple key items to consider when dealing with dispersions-
Residence time. Fluids that have particles dispersed throughout the liquid matrix have a residence time before they fall out of suspension. Have the fluid sit for too long and you will be coating a liquid without the functional component while it sits in the bottom of a pan or a slot die manifold that is too large for the application conditions.
Agglomeration. Particles can stick to each other if the initial mixing of the fluid with particles is done improperly, but it can also occur because of the coating process. Depending on the size and type of particles and the fluid and the loading of the particles, initial agglomeration can be avoided with proper mixing. Proper mixing may mean aggressive mills or simple agitation. Once the particles have been dispersed, they need to stay dispersed, so either movement, agitation or chemical separators should be considered. For shear sensitive particles, excessive agitation or the shear of too narrow a slot gap can cause a agglomeration.
In each of these instances, the simulation of the flow of the dispersion through the system can provide insight into the excessive residence time, the shear zones or other areas of concern for agglomeration or particles falling out of suspension. So whether you are dealing with beer or mayonnaise, if the fluid is loaded with dispersed particles, dig deeper to understand the material reaction to the process and the design behind the equipment to ensure precise coating capability.