Granular material is vertically vibrated in a 2D container: above a critical shaking strength, and for a sufficient number of beads, a crystalline cluster is elevated and supported by a dilute gaseous layer of fast beads underneath. We call this phenomenon the granular Leidenfrost effect. The experimental observations are explained by a hydrodynamic model featuring three dimensionless control parameters: the energy input S, the number of particle layers F, and the inelasticity of the particle

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... llisions ". The S; F phase diagram, in which the Leidenfrost state lies between the purely solid and gas phases, shows accurate agreement between experiment and theory. Vertically shaken granular matter typically exhibits a region of reduced density just above the vibrating bottom [1] [2] [3] [4] [5] . An exceptionally strong form of this so-called density inversion was recently encountered in a theoretical study by Meerson et al. [6]: for sufficiently strong shaking a dense cluster of particles, showing a hexagonal packing, was observed to be elevated and supported by a dilute layer of fast particles underneath. Here we present the first experimental observation of this phenomenon, which we will call the granular Leidenfrost effect. It is analogous to the original Leidenfrost effect of a water droplet hovering over a hot plate [7, 8] : when the temperature of the plate exceeds the Leidenfrost temperature T L 220 C (equivalent to the critical shaking strength in the granular system), the bottom layer of the drop vaporizes instantly and prevents direct heat transfer from the plate to the drop, causing the droplet to hover and survive for a long time. We also give a theoretical explanation in the spirit of Meerson et al. [6, 9, 10] . These authors focused on the point where the density at the bottom first becomes inverted, which is a precursor to the granular Leidenfrost effect (not yet the actual phase separation). We study the subsequent transition from this density-inverted state to the Leidenfrost state in which the solid and gas phases coexist. A major challenge in granular research today is to achieve a hydrodynamiclike continuum description [11] [12] [13] [14] [15] [16] , which, however, in many cases breaks down due to the tendency of the particles to cluster together [17, 18] . We show that the Leidenfrost effect (despite the clustered phase) is well described by a hydrodynamic model. Our experimental setup (Fig. 1) consists of a quasi-2D container (10 0:45 14 cm) [19] filled with glass beads of diameter d 4:0 mm, density 2:5 g=cm 3 , and coefficient of normal restitution e 0:95. The setup is mounted on a shaker with tunable frequency f and amplitude a. The Leidenfrost effect, see Fig. 1 , is stably reproduced for given, sufficiently large values of the shaking strength and the number of particle layers. The four natural dimensionless control parameters to analyze the experiment are (i) the shaking acceleration (with g the gravitational acceleration): (1) (ii) the number of bead layers F, (iii) the dimensionless shaking amplitude A a=d, and (iv) the inelasticity parameter " 1 ÿ e 2 . First the dependence on ÿ is investigated for a fixed number of layers F 16. Figures 2(a) and 2(b) show an experimental snapshot and the corresponding density profile ny (determined by counting the number of black pixels in each horizontal row) at moderate shaking, ÿ 7:7. The snapshot shows a hexagonal packing and this is reflected in the periodic structure of ny; i.e., the particles behave like a solid crystal. The theoretical profile in Fig. 2 (c) does not show this periodicity, reflecting the continuum (nonparticulate) character of the model. At vigorous shaking [Figs. 2(d) and 2(e)] the Leidenfrost state is observed: a crystalline cluster floats on top of a FIG. 1 . Granular Leidenfrost effect: glass beads, vertically vibrated above a critical shaking strength, form a crystalline cluster that is elevated and supported by a vaporlike layer of fast particles underneath. The thickness of the dilute layer oscillates in time (never vanishing) due to the motion of the bottom, while the cluster floats steadily at the same position.