Reversible Self-Assembly (fcc-bct) Crystallization of Confined Granular Spheres via a Shear Dimensionality Mechanism.

By combining vibrational annealing and shear dimensionality, we experimentally show (1) a fast reversible crystallization fcc-bct (face-centered cubic-body-centered tetragonal) in a granular system that is composed of dissipative millimeter-sized dry spheres, (2) a two-dimensional (planar) shear promotes self-assembly into an fcc crystal, while one-dimensional shear produces a bct crystal, and (3) in analogy with heterogeneous nucleation, a granular temperature gradient leads to the formation of crystal domains showing competition of polymorphic phases in the cold regions. Our findings suggest that controlling the directionality of the interactions steers to reversible crystallization of hard spheres, adds clues for theoretical studies, and provides a novel mechanism for the technological development of the applications of self-assembling phononic crystals.

Dynamic self-assembly of non-Brownian spheres studied by molecular dynamics simulations.

Granular self-assembly of confined non-Brownian spheres under gravity is studied by molecular dynamics simulations. Starting from a disordered phase, dry or cohesive spheres organize, by vibrational annealing, into body-centered-tetragonal or face-centered-cubic structures, respectively. During the self-assembling process, isothermal and isodense points are observed. The existence of such points indicates that both granular temperature and packing fraction undergo an inversion process that may be in the core of crystal nucleation. Around the isothermal point, a sudden growth of granular clusters having the maximum coordination number takes place, indicating the outcome of a first-order phase transition. We propose a heuristic equation that successfully describes the dynamic evolution of the local packing fraction in terms of the local granular temperature, along the entire crystallization process.

Free-energy landscapes of granular clusters grown by magnetic interaction.

We experimentally study the aggregation of small clusters made of non-Brownian dipolar beads in a vibro-fluidized system. The particles are paramagnetic spheres that add around a fixed magnetic seed inside a granular gas of glass beads. We observe that under appropriate physical conditions symmetric and asymmetric cluster configurations are created and, as the number of particles increases, the aggregation time obeys a power law. We use an ensemble statistics to evaluate the free-energies and entropies landscapes of the granular clusters. The correspondence between such landscapes shows that, even if the system is of macroscopic scale and not in strict equilibrium, our approach to understand the relationship between the cluster structures and the interactions that create them is reliable.

Acoustic gaps in a chain of magnetic spheres.

Acoustic gaps are normally observed in granular inhomogeneous structures made of composite materials. The modulation of the elastic properties in such media creates the coherent effects of scattering and interference that ultimately lead to frequency intervals where sound propagation is forbidden. Contrastingly, we report here an experimental observation of acoustic gaps in homogeneous media; specifically, in granular chains. The beads used in our study are magnetic. Therefore, instead of modulating the elastic properties of the chain, we modulate the magnetization (i.e., the contact forces). We also observe that the propagation speed of acoustic signals through the magnetic chains used in this study is at odds with the speed predicted by Hertz's law.

Self-assembling of dry and cohesive non-Brownian spheres.

Numerous experimental and computational studies have been carried out in recent years to understand the mechanisms governing the compaction of granular systems. Here the problem is further investigated from a different perspective. We compact spheres by a vibrational annealing method and show how the interactions between them and the walls determine the final structure. Dry spheres self-assemble only in body-centered-tetragonal structures, while cohesive ones surpass such density and reach the most compact face-centered-cubic phase. We argue that such polymorphism is due to a molecularlike behavior induced by a compensation mechanism between free and vibrational energies.

Crystallization of confined non-Brownian spheres by vibrational annealing.

We introduce an experimental method to crystallize ensembles of non-Brownian spheres confined in narrow containers. The method is based on programmed vibrations and a cooling procedure (annealing). Starting with a granular gas, the system slowly relaxes into a solid ordered structure: Body-centered-tetragonal and face-centered-cubic single crystals are obtained depending on the dimensions of the capillaries. Dry and lubricated beads behave differently, indicating that a sticking coefficient between the particles is important in the dynamics of the crystallization.