If the material is very fine and compressible then it may become fluid during filling of a process vessel this fluid is not persistent as it would be in a fluid bed where there is an external source of air the material begins to lose its entrained air soon after completion of the filling process however these material retain their fluid-like behavior for several minutes or even hours coarse particles entering the bin during this time can impact on this fluid material and penetrate the top layer of fluid solid before coming to rest below the top surface this result in a top-to-bottom segregation of particles in the bulk mixture thereby creating layers of fines andcoarse