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.