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