Role of interparticle forces in the formation of random loose packing

Phase transitions in vibrated granular systems in microgravity

We numerically investigated various dynamical behaviors of a vibrated granular gas in microgravity. Using the parameters of an earlier Mini-Texus 5 experiment, three-dimensional simulations, based on molecular dynamics, efficiently reproduce experimental results. Using Kolmogorov-Smirnov tests, four dynamical regimes have been distinguished: gaseous state, partial clustering, complete clustering, and bouncing aggregates. Different grain radii and densities have been considered in order to describe a complete (r,η)-phase diagram. The latter exhibits rich features such as phase transitions and triple points. Our work emphasizes the complexity of diluted granular systems and opens fundamental perspectives.

Defining random loose packing for nonspherical grains

The concept of "random loose packing" (RLP) has evolved through extensive study of loose packings of spheres, which has resulted in an accepted definition as the loosest packing that can be obtained by pouring grains. We extend this consideration to packings of nonspherical grains (ellipsoids) formed by slow settling in a viscous liquid, and perform a detailed analysis of the structural properties of the resulting packings. We find that as in the case of spheres the loosest ellipsoid packings are generated for grains with high interparticle friction. However, unlike spheres, these packings cannot be considered random as they have a significant degree of orientational ordering that increases with the grain's aspect ratio. This demonstrates that applying sedimentation or pouring techniques that have become part of the commonly held definition of RLP, will not generate random packings of ellipsoids. The consequences for the accepted definition of RLP and its applicability to nonspherical grains is discussed.

Density profiles of loose and collapsed cohesive granular structures generated by ballistic deposition

Loose granular structures stabilized against gravity by an effective cohesive force are investigated on a microscopic basis using contact dynamics. We study the influence of the granular Bond number on the density profiles and the generation process of packings, generated by ballistic deposition under gravity. The internal compaction occurs discontinuously in small avalanches and we study their size distribution. We also develop a model explaining the final density profiles based on insight about the collapse of a packing under changes of the Bond number.

Size-dependent multiple twinning in nanocrystal superlattices

We report a size-dependent change in the morphology of superlattices self-assembled from monodisperse colloidal PbS nanocrystals. Superlattices of large (>7 nm) PbS nanocrystals showed a strong tendency to form multiply twinned face-centered cubic superlattices with decahedral and icosahedral symmetry, exhibiting crystallographically forbidden five-fold symmetry elements. On the other hand, superlattices of small (<4 nm) PbS nanocrystals exhibited no twinning. To explain such a dramatic difference in the twinning probability, we showed that twinning energy in a nanocrystal superlattice is strongly size-dependent. In addition, the interparticle potentials acting during the self-assembly process are "softer" in the case of larger PbS nanocrystals, thus favoring the formation of multiply twinned superlattices. Our work introduces a new class of materials exhibiting multiple twinning, while offering flexibility in designing interparticle potentials.

Three-dimensional reconstruction of statistically optimal unit cells of polydisperse particulate composites from microtomography

In this paper, we present a systematic approach for characterization and reconstruction of statistically optimal representative unit cells of polydisperse particulate composites. Microtomography is used to gather rich three-dimensional data of a packed glass bead system. First-, second-, and third-order probability functions are used to characterize the morphology of the material, and the parallel augmented simulated annealing algorithm is employed for reconstruction of the statistically equivalent medium. Both the fully resolved probability spectrum and the geometrically exact particle shapes are considered in this study, rendering the optimization problem multidimensional with a highly complex objective function. A ten-phase particulate composite composed of packed glass beads in a cylindrical specimen is investigated, and a unit cell is reconstructed on massively parallel computers. Further, rigorous error analysis of the statistical descriptors (probability functions) is presented and a detailed comparison between statistics of the voxel-derived pack and the representative cell is made.

Liquid-to-solid transition of concentrated suspensions under complex transient shear histories

Suspensions containing noncolloidal aluminum particles in a Newtonian carrier liquid and the effects of shear history on the rheological properties of suspensions are investigated showing a critical concentration phi(c), which is close to the "freezing" packing fraction of spherical particle suspensions. An apparent liquid-to-solid transition was clearly found for suspensions below phi(c) via transient step shear after long shear history as well as via the large amplitude oscillatory shear flow with controlled shear stress. The microstructures or the local flow characteristics of suspensions with phi<phi(c) and with phi>phi(c) are still different according to the Fourier Transform Rheological analysis. Although suspensions with phi<phi(c) show solidlike behaviors after long shear history, the flow behaviors keep unchanged as shear stress increases, and typically show solidlike, liquid-solid coexistence, liquidlike, and jamming behaviors, successively. However, liquidlike regime of concentrated suspensions with phi>phi(c) disappears after preshear.

Random very loose packings

We measure the number Omega(phi) of mechanically stable states of volume fraction phi of a granular assembly under gravity. The granular entropy S(phi)=logOmega(phi) vanishes both at high density, at phi approximately equal to phi_rcp, and a low density, at phi approximately equal to phi_rvlp, where phi_rvlp is a new lower bound we call random very loose pack. phi_rlp is the volume fraction where the entropy is maximal. These findings allow for a clear explanation of compaction experiments and provide the first first-principle definition of the random loose volume fraction. In the context of the statistical mechanics approach to static granular materials, states with phi<phi_rlp are characterized by a negative temperature.

Computer simulation of model cohesive powders: plastic consolidation, structural changes, and elasticity under isotropic loads

The quasistatic behavior of a simple two-dimensional model of a cohesive powder under isotropic loads is investigated by discrete element simulations. We ignore contact plasticity and focus on the effect of geometry and collective rearrangements on the material behavior. The loose packing states, as assembled and characterized in a previous numerical study [Gilabert, Roux, and Castellanos, Phys. Rev. E 75, 011303 (2007)], are observed, under growing confining pressure P , to undergo important structural changes, while solid fraction Phi irreversibly increases (typically, from 0.4-0.5 to 0.75-0.8). The system state goes through three stages, with different forms of the plastic consolidation curve, i.e., Phi as a function of the growing reduced pressure P;{*}=PaF_{0} , defined with adhesion force F0 and grain diameter a . In the low-confinement regime (I), the system undergoes negligible plastic compaction, and its structure is influenced by the assembling process. In regime II the material state is independent of initial conditions, and the void ratio varies linearly with lnP [i.e., Delta(1Phi)=lambdaDelta(lnP;{*}) ], as described in the engineering literature. Plasticity index lambda is reduced in the presence of a small rolling resistance (RR). In the last stage of compaction (III), Phi approaches an asymptotic, maximum solid fraction Phi_{max} , as a power law Phi_{max}-Phi proportional, variant(P;{*});{-alpha} , with alpha approximately 1 , and properties of cohesionless granular packs are gradually retrieved. Under consolidation, while the range xi of fractal density correlations decreases, force patterns reorganize from self-balanced clusters to force chains, with correlative evolutions of force distributions, and elastic moduli increase by a large amount. Plastic deformation events correspond to very small changes in the network topology, while the denser regions tend to move like rigid bodies. Elastic properties are dominated by the bending of thin junctions in loose systems. For growing RR those tend to form particle chains, the folding of which, rather than tensile ruptures, controls plastic compaction.

Onset of mechanical stability in random packings of frictional spheres

Using sedimentation to obtain precisely controlled packings of noncohesive spheres, we find that the volume fraction phiRLP of the loosest mechanically stable packing is in an operational sense well defined by a limit process. This random loose packing volume fraction decreases with decreasing pressure p and increasing interparticle friction coefficient mu. Using x-ray tomography to correct for a container boundary effect that depends on particle size, we find for rough particles in the limit p-->0 a new lower bound, phiRLP=0.550+/-0.001.

Model for cage formation in colloidal suspension of laponite

In this paper we investigate glass transition in aqueous suspension of synthetic hectorite clay, laponite. We believe that upon dispersing laponite clay in water, the system comprises of clusters (agglomerates) of laponite dispersed in the same. Subsequent osmotic swelling of these clusters leads to an increase in their volume fraction. We propose that this phenomenon is responsible for slowing down of the overall dynamics of the system. As clusters fill up the space, the system undergoes glass transition. Along with the mode coupling theory, the proposed mechanism rightly captures various characteristic features of the system in the ergodic regime as it approaches glass transition.

Novel instrument to characterize dry granular materials at low consolidations

The performance of traditional instruments for measuring the flow properties of dry granular materials at small consolidation stresses is not fully satisfactory. Generally, commercial quick tests, as, for example, the angle repose method, do not yield intrinsic material properties. This difficulty is solved in currently available ring shear testers, in which the externally applied torque necessary for shearing the sample is measured as a function of the normal stress previously applied through an annular lid. In this article we show a novel device in which the shear stress is caused by the action of a centrifugal force on a vertical layer of unconsolidated material, which is rotated around its vertical axis. At a critical point the shear stress is large enough to drive material avalanches. From a theoretical analysis of these avalanches based on Coulomb's method of wedges, we derive the angle of internal friction and cohesion of the granular material. To illustrate the functioning of the instrument, measurements on steel, ferrite, and magnetite beads of different particle size are presented. The data obtained are used to analyze the gravity-driven avalanches of these materials in a slowly rotated drum.

Self-assembly of particles for densest packing by mechanical vibration

It is shown that by properly controlling vibrational and charging conditions, the transition from disordered to ordered, densest packing of particles can be obtained consistently. The method applies to both spherical and nonspherical particles. For spheres, face centered cubic packing with different orientations can be achieved by monitoring the vibration amplitude and frequency, and the structure of the bottom layer, in particular. The resultant force structures are ordered but do not necessarily correspond to the packing structures fully. The implications of the findings are also discussed.