Doublet craters originated by low speed impact experiments in granular matter

We studied the formation of doublet craters generated by binary projectiles (steel beads or granular agglomerates) impacting simultaneously at low speed against a sand bed. For impacts of two solid beads, we observed different morphologies depending on the initial separation between the projectiles: two well defined craters, overlapping craters, or a single larger circular crater produced when the separation between projectiles is negligible. Doublet craters are characterized by an extended ridge between the depressions produced by the collision of the ejected curtains. When there is a slight delay between both impacts, the ridge and the ejecta curtains are not symmetric, and the delayed projectile penetrates deeper than its partner due to the bed fluidization and material displacement induced by the first impact. For experiments with sand agglomerates that are destroyed by the collision, doublets are also generated, but their morphology is considerably affected by the ejection of the projectiles debris. The resulting morphologies are similar to those found in numerical simulations (Miljković et al. in Earth Planet Sci Lett 363:121–132, 2013. https://doi.org/10.1016/j.epsl.2012.12.033) and experiments reported at considerably higher impact energies (Oberbeck and Aoyagi in J Geophys Res (1896–1977) 77(14):2419–2432, 1972. https://doi.org/10.1029/JB077i014p02419; Oberbeck in Moon 6(1–2):83–92, 1973. https://doi.org/10.1007/BF02630653), showing that the main features of doublet impact cratering can also be reproduced at low-energy scale.