Reversing the Brazil-Nut Effect: Competition between Percolation and Condensation

We report on experiments on vertically shaken binary granular mixtures, which separate into their components due to the external excitation. This well-known phenomenon, where large particles rise to the top of the mixture, is called the Brazil-nut effect. Recent theoretical findings predict also a reverse Brazil-nut effect, where large particles sink to the bottom of the container. We choose spherical beads of various diameters and materials in order to observe the transition from Brazil-nut effect to its reverse form. The direction of demixing depends sensitively on the external excitation, so that it is possible to switch between both effects for a given mass density ratio.

Figure 1

Temporal evolution of the system. (a) Initially 8 mm glass beads on top of 15 mm polypropylene, which show the classical Brazil-nut effect. (b) 10 mm bronze spheres on 4 mm glass beads showing the reverse Brazil-nut effect.

Figure 2

Phase space for particle properties. The plot shows the regimes where reverse (RBN) and classical Brazil-nut effect (BN) occurs depending on the particle properties. The solid line separating both areas is given by expression (1). Each symbol represents one experiment. Symbols on top of each other represent two different particle mixtures with the same diameter and density ratio, but show different behaviors.

Figure 3

Particle behavior for different driving modes of the shaker. The mixtures used are 10 mm polyurethane and 4 mm glass spheres. Lines are drawn to guide the eye.