Because of the effects of surface tension some powdered materials such as fly ash, PAC, and Bentonite, will not immediately sink below the surface of the fluid when added to a tank. If feeding too fast, material can build up on the surface forming clumps that are difficult to mix, and eventually completely blocking material from being entrained.
(An everyday example of this effect can be found in coffee, if you add sugar to your coffee it immediately drops below the surface (easy to wet) however, if you are adding powdered cream this will tend to float on the surface for a period before submerging (difficult to wet). There are several different strategies for pre-wetting.
The solution we recommend will depend on the specific material the feed rate and desired concentration.