  Corridors of size-selected crescent-shaped dunes, known as barchans, are
commonly found in water, air, and other planetary environments. The growth of
barchans results from the interplay between a fluid flow and a granular bed,
but their size regulation involves intricate exchanges between different
barchans within a field. One size-regulating mechanism is the binary
interaction between nearby dunes, when two dunes exchange mass via the near
flow field or by direct contact (collision). In a recent Letter (Assis and
Franklin, Geophys. Res. Lett., 2020), we identified five different patterns
arising from binary interactions of subaqueous barchans, and proposed
classification maps. In this paper, we further inquire into binary exchanges by
investigating the motion of individual grains while barchans interact with each
other. The experiments were conducted in a water channel where the evolution of
pairs of barchans in both aligned and off-centered configurations was recorded
by conventional and high-speed cameras. Based on image processing, we obtained
the morphology of dunes and motion of grains for all interaction patterns. We
present the trajectories of individual grains, from which we show the origin
and destination of moving grains, and their typical lengths and velocities. We
also show that grains from the impacting dune spread with a diffusion-like
component over the target barchan, and we propose a diffusion length. Our
results provide new insights into the size-regulating mechanisms of barchans
and barchanoid forms found on Earth and other planets.
