Although experimentalists suspect some effect of the Relative Humidity (RH) on the physical properties of granular materials, almost no author reports the value of RH besides his data. This missing information is however crucial, as explained in our last paper published in Phys. Rev. E.
In order to quantify the effect of RH, we performed a series of compaction experiments under controled RH. Even large glass beads (diameter 1 mm) have been used, a significant effect of RH is observed on the compaction dynamics. Dry air implies triboelectricity. Moisture implies nucleation of liquid bridges. Cohesion becomes therefore important for low and large RH values. An optimum, reducing cohesion, is found around RH=45%.
N.Vandewalle et al, Phys. Rev. E 85, 031309 (2012) – PDF