An assembly of opposing discs or cones for a disperser where each disc or cone has mounted to it a plate or an array of plate segments with a front surface on each plate or array of plate segments and each surface has a series of bars, grooves and dams. The bars are in rows, with the rows separated by annular dams at substantially fixed radial locations, and the grooves are adjacent the bars in the rows so that the grooves form a serpentine passage extending radially between the opposing plates or array of plate segments on the opposing discs or cones. The opposing plate or array of plate segments are arranged such that the annular dams on one plate or array of plate segments face roughly the middle of the row of bars on the opposing plate or array of plate segments.