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

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 ( $Ad=\omega/2\rho_pU^2_0R$). Four regimes are identified based on the value of $Ad$: RCP regime with $Ad<\sim 0.01$; RLP regime with $\sim 0.01<Ad<1$; adhesion regime with $1<Ad<20$ and an asymptotic regime with $Ad>20$. Force distribution of these adhesive loose packings follows $P(f)\sim f^\theta$ for small forces and $P(f)\sim \exp^{-\beta f}$ for big forces, respectively, which shares a similar form with that in packings without adhesion but results in distinct exponents of $\theta=0.879$, $\beta=0.839$. 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.