Presentation of the pattern formation mechanism. The liquid flows down the wall (represented by the arrow in 2a), mostly in the regions where the constraint due to the particles (sticking to the wall) is weaker. Since the fluid mostly flows in these regions, some particles are detached from the wall (shaded in 2b), the concentration decreases and the constraint too. Therefore the flow increases in these regions (thicker arrow in 2c) and the phenomenon is amplified.