Role of friction in granular Plateau-Rayleigh instability.*

Streams of weakly cohesive granular materials exhibit liquid-like breakup into droplets. We numerically study such a granular analog of the Plateau-Rayleigh instability in a free-falling granular stream. We use a discrete element method (DEM) simulation with Johnson-Kendall-Robert (JKR) interactions between the grains, as well as sliding and rolling frictions. Our simulation results recover the experimentally observed w_min∝(t - t₀ )^2/3  for large (t - t₀), where w_min is the minimum width of the neck and t₀ is the rupture time. Interestingly, no breakup is observed for frictionless grains. This suggests that friction and the motion tangential to the stream direction play an important role in the granular Plateau-Rayleigh instability.