More than one coater can be used. A sheet can be coated with a solution that includes pressure rupturable materials, or tiny capsules that rupture when put under pressure. An example is carbonless paper. One of the papers is coated with a material that has a chemical captured inside tiny capsules that rupture when a pen is drawn over them. A second sheet is then coated with a matierla that reactions to that chemical by changing colors.
Rheology is the science of studying the friction between liquids.
In papermaking, it is mainly used in studying the bahavior of coatings. Paperboard is a heavy layered paper, usually at least 100 lbs per ream or more. It is intended to be a rigid, durable form of paper, often used in packaging. It is usually the inside layer of a 3 layer sheet. The broke is usually recycled in a Hydrapulper .