Computer simulation of random loose packings of micro-particles in presence of adhesion and friction

Abstract

With a novel 3D discrete-element method specially developed with adhesive contact mechanics, random loose packings of uniform spherical micron-sized particles are fully investigated. The results show that large velocity, large size or weak adhesion can produce a relatively dense packing when other parameters are fixed, and these combined effects can be characterized by a dimensionless adhesion parameter ( Ad = ω/2ρpU 2 0 R). Four regimes are identified based on the value of Ad: RCP regime with Ad <∼ 0.01; RLP regime with ∼ 0.01 < Ad < 1; adhesion regime with 1 < Ad < 20 and an asymptotic regime with Ad > 20. Force distribution of these adhesive loose packings follows P (f ) ∼ f θ for small forces and P (f ) ∼ exp −βf for big forces, respectively, which shares a similar form with that in packings without adhesion but results in distinct exponents of θ = 0.879, β = 0.839. A local mechanical equilibrium analysis shows that adhesion enhances both sliding and rolling resistance so that fewer neighbours are needed to satisfy the force and torque balance.

A fast adhesive discrete element method for random packings of fine particles

“…After normalizing F N in each case with the corresponding mean value of its magnitude, < |F N | >, distributions with different χ nicely collapse onto a single curve. The normalized distributions are almost symmetrical around F N / < |F N | >= 0, which is in good agreement with previous results on the packing of strong adhesive particles Liu et al (2016b). The results again verify that the fast adhesive DEM with reduced particle stiffness can retain both the structural and mechanical properties of the contact network in a packing.…”

“…A proper description of adhesive rolling and sliding resistances is of significance to predict the formation of agglomerates and the structure of particle deposits. For adhesive microparticles, rolling is generally the preferred deformation mode, which gives rises to the rearrangement of packing structures (Dominik, 1997;Liu et al, 2016bLiu et al, , 2017b. To accurately simulate the rolling motion, the adhesive rolling model needs to be modified in the framework of JKR-based DEM with reduced stiffness.…”

Random loose packing of small particles with liquid cohesion

“…The packing generation method applied here is the sequential adding protocol, which can reproduce a random ballistic deposition process of microparticles in the nature, that is, particles sequentially enter the cuboid chamber with an initial velocity U 0 and then fall under the effect of gravity. This protocol for microparticle system is dominated by the particle inertia (kinetic energy) and adhesion/cohesion . The gravitational acceleration is fixed in x direction and g = 9.81 m/s 2 , and the deposition plane in the container is of an area of L × L = 20 r p × 20 r p , such that the ratio of the particle diameter to the width of container is obtained as 0.1.…”

“…However, the definition of RLP is still in controversy compared to that of RCP, especially for dry micron‐sized particles, due to the presence of interparticle van der Waals and electrostatic forces. A survey of literature indicates that the random packing of fine particles with van der Waals and electrostatic forces significantly differs from RCP and RLP . Recently, we reported that the packing fraction of adhesive microparticles can be achieved even as small as 0.125 in a random ballistic deposition process, which is identified as the adhesive loose packing (ALP) .…”

“…A survey of literature indicates that the random packing of fine particles with van der Waals and electrostatic forces significantly differs from RCP and RLP . Recently, we reported that the packing fraction of adhesive microparticles can be achieved even as small as 0.125 in a random ballistic deposition process, which is identified as the adhesive loose packing (ALP) . Therefore, interparticle force plays an important role in producing various structures of random packing of grains.…”