Granular jets

Worthington jets are a familiar sight in light rain upon puddles and ponds. These narrow vertical jets are formed by the radial collapse of the liquid "craters" produced by the impacting rain drops. Such jets can also be generated by super-critically forcing the standing Faraday waves on a liquid surface and have recently been cast in the formalism of physical singularities to investigate the role of the inertial focusing and the influence of surface tension on their strength. Zeff et al. propose that during the collapse of the free-surface cavity, the surface develops a curvature singularity which is dominated by inertia and surface tension, hence generating high-energy vertical jets. We have discovered that similar narrow jets occur even for granular materials, where surface tension is absent. This new phenomenon suggests that a singularity in the surface tension force is not needed to produce such jets and raises the question whether the inertial focusing is the sole mechanism. . . .published or submitted for publicationis peer reviewe