An unexplored regime of binary packing under extreme mixture conditions

Abstract

We present an unexplored regime, where the binary random close packing fraction ϕRCPb is smaller than that of the mono-sized one ϕRCPm. This is against previous observations and common perceptions that binary packing tends to be denser than mono-sized packing. We numerically confirm the critical condition for reaching this exceptional regime in the size ratio (Rr) and mole fraction (Xs) space, where Rr is close to 1, and the mole fraction of the smaller sphere Xs close to 0. Under the same loading condition, the stiffness of the packing at this exceptional regime is found to be significantly higher than that of the mono-sized packing. The formation and transition of this regime for varying Rr and Xs are theoretically modelled based on the hard-sphere fluid theory. This exceptional regime remains unreported in existing literature, yet significant for our fundamental understanding of binary packing systems.