  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.
