Adhesive loose packings of small dry particles

Abstract

We explore adhesive loose packings of dry small spherical particles of micrometer size using 3D discrete-element simulations with adhesive contact mechanics. A dimensionless adhesion parameter (Ad) successfully combines the effects of particle velocities, sizes and the work of adhesion, identifying a universal regime of adhesive packings for Ad > 1. The structural properties of the packings in this regime are well described by an ensemble approach based on a coarse-grained volume function that includes correlations between bulk and contact spheres. Our theoretical and numerical results predict: (i) An equation of state for adhesive loose packings that appears as a continuation from the frictionless random close packing (RCP) point in the jamming phase diagram; (ii) The existence of a maximal loose packing point at the coordination number Z = 2 and packing fraction φ = 1/2 3 . Our results highlight that adhesion leads to a universal packing regime at packing fractions much smaller than the random loose packing, which can be described within a statistical mechanical framework. We present a general phase diagram of jammed matter comprising frictionless, frictional, adhesive as well as non-spherical particles, providing a classification of packings in terms of their continuation from the spherical frictionless RCP.Jammed particle packings have been studied to understand the microstructure and bulk properties of liquids, glasses and crystals [1, 2] and frictional granular materials [3,4]. Two packing limits have been identified for disordered uniform spheres: The random close packing (RCP) and random loose packing (RLP) limits [1,[5][6][7][8][9][10][11]. The upper RCP limit is reproduced for frictionless spheres at volume fractions φ ≈ 0.64 and has been associated with a freezing point of a 1st order phase transition [12][13][14][15], among other interpretations [2,16]. In the presence of friction, packings reach lower volume fraction up to the RLP limit φ RLP ≈ 0.55 for mechanically stable packings [6,8,11]. However, most packings of dry small micrometer-sized particles in nature are not only subject to friction, but also adhesion forces. In fact, van der Waals forces generally dominate interactions between particles with diameters of around 10µm or smaller. In this case, the adhesive forces begin to overcome the gravitational and elastic contact forces acting on the particles and change macroscopic structural properties [17,18].Despite the ubiquity of adhesive particle packings in almost all areas of engineering, biology, agriculture and physical sciences [18][19][20][21], these packings have so far not been systematically investigated. The multi-coupling of adhesion, elastic contact forces and friction within the short-range particle-particle interaction zone and their further couplings with fluid forces (e.g., buoyancy, drag and lubrication) across long-range scales make it highly difficult to single out the effect of the adhesion forces * lishuiqing@tsinghua.edu.cn † hmakse@lev.ccny.cuny.edu alone. Previous stud...

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