  Barchans are dunes of crescentic shape found on Earth, Mars and other
celestial bodies, growing usually on polydisperse granular beds. In this
Letter, we investigate experimentally the growth of subaqueous barchans
consisting of bidisperse grains. We found that the grain distribution within
the dune changes with the employed pair, and that a transient stripe appears on
the dune surface. We propose that observed patterns result from the competition
between fluid entrainment and easiness of rolling for each grain type, and that
grains segregate with a diffusion-like mechanism. Our results provide new
insights into barchan structures found in other environments.
