We fluidize a granular bed in a rectangular container by injecting energy through the lateral walls with high-frequency sinusoidal horizontal vibrations. In this way, the bed is brought to a steady state with no convection. We measured buoyancy forces on light spheres immersed in the bed and found that they obey Archimedes' principle. The buoyancy forces decrease when we reduce the injected energy. By measuring ascension velocities as a function of gamma, we can evaluate the frictional drag of the bed; its exponential dependence agrees very well with previous findings. Rising times of the intruders ascending through the bed were also measured, they increase monotonically as we increase the density.

Energetic collisions of subatomic particles with fixed or moving targets have been very valuable to penetrate into the mysteries of nature. But the mysteries are quite intriguing when projectiles and targets are macroscopically immense. We know that countless debris wandering in space impacted (and still do) large asteroids, moons and planets; and that millions of craters on their surfaces are traces of such collisions. By classifying and studying the morphology of such craters, geologists and astrophysicists obtain important clues to understand the origin and evolution of the Solar System. This review surveys knowledge about crater phenomena in the planetary science context, avoiding detailed descriptions already found in excellent papers on the subject. Then, it examines the most important results reported in the literature related to impact and penetration phenomena in granular targets obtained by doing simple experiments. The main goal is to discern whether both schools, one that takes into account the right ingredients (planetary bodies and very high energies) but cannot physically reproduce the collisions, and the other that easily carries out the collisions but uses laboratory ingredients (small projectiles and low energies), can arrive at a synergistic intersection point.

An object falling in a fluid reaches a terminal velocity when the drag force and its weight are balanced. Contrastingly, an object impacting into a granular medium rapidly dissipates all its energy and comes to rest always at a shallow depth. Here we study, experimentally and theoretically, the penetration dynamics of a projectile in a very long silo filled with expanded polystyrene particles. We discovered that, above a critical mass, the projectile reaches a terminal velocity and, therefore, an endless penetration.

Multilinear regression has been used extensively to predict soil hydraulic properties, both the θ(h) and K(h) relationships, from easily obtainable soil variables. As an alternative, this study investigated the performance of an artificial radial basis neural network in predicting some K(h) values from other variables. This kind of neural network may be seen as a multivariate interpolation technique, which can theoretically fit any nonlinear continuous function. Neural networks are characterized by parameters that must be optimized to solve a given problem. We used a fitting procedure requiring only two parameters to ensure a unique solution. These two parameters were determined by data splitting. Hypothetical data bases with uncertainties were simulated to analyze the performance of the neural network in predicting a nonlinear relation derived from a classical model for K(h). A soil data base covering a broad spectrum of soil horizons was used to test the neural network in solving multivariate problems. Numerical simulations showed that the neural network was sensitive to large uncertainties in the data base. It was more efficient than a multilinear regression when the uncertainties were small. Experimental results showed that the neural network was more efficient than the multilinear regression for predicting K(h = −1 m) or K(h = −2.5 m) from two qualitative and five quantitative soil variables. It was also more efficient than two independent multilinear regressions, one for the sandy samples and the other for the loamy and clayey samples. Provided that a large data base with accurate K values is available, artificial neural networks can be useful to predict θ(h) and K(h) over a broad spectrum of soils.

The segregation of large spheres in a granular bed under vertical vibrations is studied. In our experiments we systematically measure rise times as a function of density, diameter and depth; for two different sinusoidal excitations. The measurements reveal that: at low frequencies, inertia and convection are the only mechanisms behind segregation. Inertia (convection) dominates when the relative density is greater (less) than one. At high frequencies, where convection is suppressed, fluidization of the granular bed causes either buoyancy or sinkage and segregation occurs.PACS numbers: 46.10.+z, 64.75.+g, 83.80.Fg Many theoretical and experimental studies have been carried out in the last five decades aimed to reveal the physics of one of the most intriguing phenomena in granular matter: vibration-induced segregation [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. However, when this phenomenon intensively investigated appeared to be well understood, new scientific puzzles came into scene [15,16,17,18,19,20,21,22]. Air-driven segregation, inertia, condensation, are now new words added to the already vast list of concepts important in the subject. Thus, since this problem is an important concern to industries dealing with granulates, these recently disclosed effects should be further investigated. Granular segregation was first reported in 1939 by Brown [23] and studied ever since by the engineering community [1,2,3,4,5], until it was brought in 1987 to the physics realm with the suggestive name of "Brazil Nut Problem" (BNP) [6]. Results related to this problem established themselves as benchmarks of granular segregation. But the question: why the Brazil nuts are on top, seems to be yet an open matter of discussion. Both theoretical and experimental studies have focused on the influence of size, friction, density and excitation parameters [7,8,9,10,12,13,16,22,26,27] and the results explain, or obscure, bit by bit the underlying mechanism behind the BNP. Some of these results support the idea that it is "void-filling" beneath large ascending particles, the mechanism promoting the upward movement of an intruder in a shaken granular bed [6,14]. Other researches claim that global convection is the driven force behind the BNP [8], and others, that arching [7] or inertia [16,22] are crucial elements to explain it . The dilemma is not yet settled on the verge of even more recent findings [15]; being the most relevant the surprising result that decreasing the density of the intruder does not necessarily mean a monotonic increasing of the rise time, as might be previously suggested by studies in 3D [9] and 2D [16]. Furthermore, based on computer simulations Hong et al even dared to predict the reverse segregation effect in the BNP [17,19] (known now in the literature as the RBNP), which was immediately confronted by two groups [20,24], but nevertheless observed in the laboratory by Breu et al [25]. Finally, Yan et al [21] recently failed to confirm the experimental findings of Möbius et al [15].Based on this debate, a simple, yet...

Some materials remain solids even if they are heated beyond the temperature of their melting points. In condensed matter physics, this rare phenomenon is called superheating. Here we report the analogous phenomenon in granular matter: a strongly vibrated monolayer that instead of being a gas persists as a crystal for some time. Eventually, it spontaneously evaporates. We found that the system has thermodynamiclike features like coexistence and metastability. We show how the observed metastable phase is linked to energy dissipation.

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The rise dynamics of a large particle, in a granular bed under vertical vibrations, is experimentally studied with an inductive device designed to track the particle while it climbs through the granulate under different conditions. A model based on energy considerations is presented to explain our experimental data, drawing the important conclusion that it is the inertia of the particle, assisted by Reynolds dilatancy, the driven force behind its ascension mechanism. The ascension reveals a friction profile within the column which remains unchanged for different accelerations.