Stokes's cradle: Newton's cradle with liquid coating.

Granular flows involving liquid-coated solids are ubiquitous in nature (pollen capture, avalanches) and industry (filtration, pharmaceutical mixing). In this Letter, three-body collisions between liquid-coated spheres are investigated experimentally using a "Stokes's cradle," which resembles the popular desktop toy Newton's cradle (NC). Surprisingly, previous work shows that every possible outcome was observed in the Stokes's cradle except the traditional NC outcome. Here, we experimentally achieve NC via guidance from a theory, which revealed that controlling the liquid-bridge volume connecting two target particles is the key in attaining the NC outcome. These three-body experiments also provide direct evidence that the fluid resistance upon rebound cannot be completely neglected due to presumed cavitation; this resistance also influences two-body systems yet cannot be isolated experimentally in such systems.

Simulations of a binary-sized mixture of inelastic grains in rapid shear flow.

In an effort to explore the rapid flow behavior associated with a binary-sized mixture of grains and to assess the predictive ability of the existing theory for such systems, molecular-dynamic simulations have been carried out. The system under consideration is composed of inelastic, smooth, hard disks engaged in rapid shear flow. The simulations indicate that nondimensional stresses decrease with an increase in d(L)/d(S) (ratio of large particle diameter to small particle diameter) or a decrease in nu(L)/nu(S) (area fraction ratio), as is also predicted by the kinetic theory of Willits and Arnarson [Phys. Fluids 11, 3116 (1999)]. Furthermore, the level of quantitative agreement between the theoretical stress predictions and simulation data is good over the entire range of parameters investigated. Nonetheless, the molecular-dynamic simulations also show that the assumption of an equipartition of energy rapidly deteriorates as the coefficient of restitution is decreased. The magnitude of this energy difference is found to increase with the difference in particle sizes.

Inelastic collapse in simple shear flow of a granular medium.

Previous investigations have shown that inelastic collapse is a common feature of inelastic, hard-sphere simulations of nondriven (or unforced) flows, provided that the coefficient of restitution is small enough. The focus of the current effort is on a driven system, namely, simple shear flow. Two-dimensional, hard-sphere simulations have been carried out over a considerable range of restitution coefficients (r), solids fractions (nu), and numbers of particles (N). The results indicate that inelastic collapse is an integral feature of the sheared system. Similar to nondriven systems, this phenomenon is characterized by a string of particles engaging in numerous, repeated collisions just prior to collapse. The collapsed string is typically oriented along a 135 degrees angle from the streamwise direction. Inelastic collapse is also found to be more likely in systems with lower r, higher nu, and higher N, as is true for unforced systems. Nonetheless, an examination of the boundary between the collapsed and noncollapsed states reveals that the sheared system is generally more "resistant" to inelastic collapse than its nondriven counterpart. Furthermore, a dimensionless number V* is identified that represents the magnitude of the initial fluctuating velocities relative to that of a characteristic steady-state velocity (i.e., the product of shear rate and particle diameter). For values of V*>>O(1), the transient portion of the simulation is found to be more reminiscent of a nondriven system (i.e., isotropic particle bunching is observed instead of diagonal particle bands).