Dust flux (left, absolute values), dust-to-gas ratio (middle), and dust grain size (right) evolution of a disc forming from a 1 M_⊙ cloud core with a centrifugal radius of 10 au and photoevaporation according to Pic21. When the disc is young and the gas expands outwards by viscous evolution, the dust in the outer disc also moves outwards as it is strongly coupled to the gas. The red line marks the radius at which the dust transitions from moving inwards to moving outwards in the disc. Once photoevaporation opens a gap in the gas, the dust interior to this line is quickly drained. The dust remaining outside the gap is slowly pushed outwards as the gas at the outside edge of the gap photoevaporates. Two islands of a dust-to-gas ratio higher than the initial value of 0.01 appear. One at 0.2–0.4 Myr at radii between 0.1 and 5 au, and one at 0.6–0.9 Myr at radii from 0.1 to 1.5 au. The particle size generally is larger in the inner regions of the disc and at earlier times.