Publications

Precursor Nuclei on the Bottom of a Vibrating Container: The Onset of Granular Self-Assembly Crystallization

Kinetic energy is transferred through collisions to the millimeter particles that are in contact with the base of a vertically vibrated 3D container. In the early stages of a vibrational annealing process where the dimensionless acceleration is kept constant, the spontaneous appearance and disappearance of unstable clusters of ordered particles near the bottom surface of the container is observed. In later stages of this vibrational annealing process, the precursor nuclei stabilize becoming stable crystal seeds which resembles a thermal phase transition. Molecular Dynamics simulations based on these experimental observations are used to study the unstable– stable transition. The Ornstein– Zernike equation using the Percus– Yevick closure is applied to the stages before and after the observation of the stable crystal seeds in order to extract the effective potentials associated with the phase transition. Both the radial distribution function and the effective potentials show a clear correspondence of the spatial correlation as the crystallization phase begins to appear.

Rotational Transport via Spontaneous Symmetry Breaking in Vibrated Disk Packings

It is shown that vibrated packings of frictional disks self-organize cooperatively onto a rotational-transport state where the long-time angular velocity ¯$ømega$i of each disk i is nonzero. Steady rotation is mediated by the spontaneous breaking of local reflection symmetry, arising when the cages in which disks are constrained by their neighbors acquire quenched disorder at large packing densities. Experiments and numerical simulation of this unexpected phenomenon show excellent agreement with each other, revealing two rotational phases as a function of excitation intensity, respectively, the low-drive (LDR) and the moderate-drive (MDR) regimes. In the LDR, interdisk contacts are persistent and rotation happens due to frictional sliding. In the MDR, disks bounce against each other, still forming a solid phase. In the LDR, simple energy-dissipation arguments are provided, that support the observed dependence of the typical rotational velocity on excitation strength.

Sustained Rotation in a Vibrated Disk with Asymmetric Supports

A single frictional elastic disk, supported against gravity by two others, rotates steadily when the supports are vibrated and the system is tilted with respect to gravity. Rotation is here studied using molecular dynamics simulations, and a detailed analysis of the dynamics of the system is made. The origin of the observed rotational ratcheting is discussed by considering simplified situations analytically. This shows that the sense of rotation is not fixed by the tilt but depends on the details of the excitation as well.

Rotation in a Gravitational Billiard

Gravitational billiards composed of a viscoelastic frictional disk bouncing on a vibrating wedge have been studied previously, but only from the point of view of their translational behavior. In this work, the average rotational velocity of the disk is studied under various circumstances. First, an experimental realization is briefly presented, which shows sustained rotation when the wedge is tilted. Next, this phenomenon is scrutinized in close detail using a precise numerical implementation of frictional forces. We show that the bouncing disk acquires a spontaneous rotational velocity whenever the wedge angle is not bisected by the direction of gravity. Our molecular dynamics (MD) results are well reproduced by event-driven (ED) simulations. When the wedge aperture angle θ_W > π/2, the average tangential velocity of the disk scales with the typical wedge vibration velocity , and is in general a nonmonotonic function of the overall tilt angle of the wedge. The present work focuses on wedges with θ_W=2π/3θ, which are relevant for the problem of spontaneous rotation in vibrated disk packings.

Self-Assembling of Non-Brownian Magnetized Spheres

If we pour spherical beads in a container and then gently shake it to increase the compaction of the system, the packing fraction will converge logarithmically to 0.64, the density of a random close packing. If the system is specially sheared, or tapped through an annealing procedure, lattices may self-organize. In this work we study granular crystallization induced by magnetic cohesion. We observe an interesting granular polymorphism probably due to an effective van der Waals-like interaction.