The present invention relates to water swellable, water insoluble superabsorbent materials having controlled variable gel-bed friction angles. It is further effective if the capacity ie of the auxiliary retaining grooves and the capacity is of the main retaining grooves are designed so as to satisfy a relation isie0. The filter assembly includes a first filter element including a mounting structure at a first axial end to attach the first filter element to the tube sheet adjacent an opening.