An Experimental Study On The Influences Of Granular Particle Facetedness And Angularity On The Packing Structures And Compaction Dynamics

Granular materials are ubiquitous in industries and daily lives.Granular particles in the natural world and industrial and agricultural communities are usually not spherical and have obvious facetedness and angularity.The facetedness and angularity of particles lead to more stable contact structures and orientational correlations between neighboring particles,which lead to packing structures and structural evolution mechanisms that are different from those of spherical or spherical-like particle systems.Therefore,the packing and structural evolution of spherical particles,which have been most widely studied in experiments and computer simulations in the past,may not be directly applicable to these particle systems with complex shapes,and further detailed experimental investigations of the packing of faceted and angular particles are necessary to be in comparison with packing of spherical ones.In this paper,we study the three-dimensional static packing structures of three types of regular polyhedral particles using magnetic resonance imaging techniques,and also study the compaction dynamics under vibration of two-dimensional packing structures of a series of rounded pentagon particles in between disks and regular pentagons using visible light imaging.By comparing the spherical(round)and non-spherical(non-round)particle packing,we analyze the effects of particle facetedness and angularity on the packing structures and structural relaxation processes.Our specific research contents are as follows.(1)We reconstructed the random packing structures of regular octahedral,dodecahedral,icosahedral and spherical particles using magnetic resonance imaging techniques and image processing algorithms.We analyzed the typical structural parameters including the volume and shapes of Voronoi cells,correlation functions of particle centroids and orientations,probabilities distributions of pore sizes and shapes.It is found that for particles with stronger asphericity,the average degree anisotropy of Voronoi cells is larger,and the positional correlation of particles is weaker while the orientation correlation is stronger.However,such particle shape effect is not obvious when it comes to the pore structure.For example,the pore size distribution and pore connectivity as quantified by the genus of pore network are only indirectly affected by the particle angularity,and generally depend on the average packing fraction alone.(2)We generated a series of random structures with the same shape and packing fraction as the particles in the experiment,but without considering the hard interaction between particles,and analyzed the critical percolation radius of the pores to study the respective effects on percolation behavior of the relative positions and orientations between particles and the packing fraction.It is found that the shape effects are obvious in both experimental and numerically-generated random packing,that is,the larger the particle asphericity,the smaller the critical pore radius.However,the critical radius of pore percolation of the experimental packing structures is different from that of numerically-generated random packing structures,which is mainly affected by the relative positions of particle centroids due to the hard interaction,while the relative orientations of particles have only relatively weak influences.(3)We made a series of rounded pentagon particles the shapes of which are in between a regular pentagon and a round.Their packing structures are reconstructed using visible light imaging and image processing algorithms,and the structural evolutions during compaction processes are studied,to explore the influence of vibration intensity on the compaction dynamics of particles with different degrees of facetedness.It turns out that the steady packing fraction of regular pentagon particles decreases with increasing of vibration intensity,and the steady-state packing structures of round particles are close to the hexagonal close-packing structure.For the rounded pentagon particles,the steady-state packing fraction is slightly less than those of their respective densest crystalline arrangements,and also decreases with increasing particle-shape roundness.