Monomer-particle types used for the photophoresis experiments. a) Monodisperse spherical black SiO₂ particles with radii of 0.5   μm (silica spheres coated with a black organic colour, adopted from Steinbach 2004). b) Irregular San-Carlos olivine particles with a radius distribution of 0.5−15   μm. c) Monodisperse spherical transparent SiO₂ particles with radii of 0.75   μm. A scale bar of 10   μm length is shown in each sub-figure.