Graphical illustration of the feedback loop. The thermal and density structure of a protoplanetary disc is determined by this loop: temperature and gas surface density affect the relative velocities for grains of different sizes. Through the relative velocities we find the outcomes of collisions between grains, therefore a grain size distribution is created. The grains are then vertically distributed according to their sizes and the turbulence strength. The spatial distribution of the grain sizes determines the opacity of the disc, which then affects its cooling rate and the stellar heating. This way the temperatureand density of the disc change and subsequently its whole structure.