Yes, that's my thesis topic. The problem with this concept (experimentally determined) is that no predicting method or model exists that could help design equipment given known parameters. It is a very old problem, many people have tackled it, but so far, no explicit, extensively used solution exists.
Now, why is this so important? To answer that question, we have to go back to the 1960's when Andrew Jenike published his now extensively cited report. But even before, people like Janssen had understood that powders did not behave like liquids when they are stored in containers. Janssen found that unlike liquids where the pressure is proportional to the height of the container (so that as the container becomes higher, the pressure at the bottom increases), the pressure increases and at some point it becomes constant so that no matter how high the silo is, the pressure will be constant at the bottom. Janssen found that the reason this is so, it's because of friction, the powder and the wall develop friction at contact, which helps in supporting the whole mass of the powder.
Then Jenike came along and used this previous findings to develop a comprehensive method for designing silos, bunkers, and any vessel that may contain powder. But wall friction is one of the main parameters needed for the selection of the correct hopper for the silo. Since the powder has to flow by gravity, the hopper has to be used in aiding the powder flow out of the silo, this hopper can have many converging geometries. The correct angle of this hopper depends strongly on the wall friction created by the contact of the powder and the wall.