Abstract: Three dimensional dunes of crescentic shape, called barchans, are commonly found on 21 Earth and other planetary environments. In the great majority of cases, barchans are 22 organized in large fields in which corridors of size-selected barchans are observed, and 23 where barchan-barchan interactions play an important role in size regulation. Previous 24 studies shed light on the interactions between barchans by making use of monodisperse 25 particles, but dunes in nature consist, however, of polydisperse grains. In this paper, we 26 investigate the binary interactions of barchans consisting of (i) bidisperse mixtures of 27 grains and (ii) different monodisperse grains (one type for each barchan). We performed 28 experiments in a water channel where grains of different sizes were poured inside form29 ing two barchans that interacted with each other while filmed by a camera, and we ob30 tained their morphology from image processing. We observed that a transient stripe ap31 pears over the dunes in cases of bidisperse mixtures, that interaction patterns vary with 32 concentrations, and that different interactions exist when each barchan consists of dif33 ferent monodisperse grains. Interestingly, we found the conditions for a collision in which 34 the upstream barchan is larger than the downstream one, and we propose a timescale 35 for the interactions of both monodisperse and bidisperse barchans. Our results repre36 sent a new step toward understanding complex barchanoid structures found on Earth, 37 Mars and other celestial bodies Ver menos