Transport and dispersing systems
Why is wetting so difficult? First of all, fluids have different surface tensions. If, for example, a small amount of water falls onto a surface, drops form. The drops are held together by cohesion forces. By contract, the adhesion forces cause the drops to spread out on a surface. The cohesion forces, therefore, prevent a fluid from wetting a surface. If the fluid wets a surface, its contact angle to the surface changes.
With powders, on the other hand, the surface has a large influence on its ability to be wetted. The surfaces of solids have different qualities, e.g. different levels of roughness. With a solid, which has been milled very fine, the individual grains have a larger surface area than a solid that has been milled less fine. These surfaces are initially wetted with air. During mixing, the air has to be replaced by the fluid. The larger the surface of the individual particles, the more difficult this is to achieve. Fine powders also frequently form agglomerates, which make dispersing more difficult.