Structural similarity between dry and wet sphere packings

The mechanical properties of granular materials change significantly in the presence of a wetting liquid which creates capillary bridges between the particles. This results e.g. in a reduced value of the volume fraction when a packing is prepared with added liquid. Here we use x-ray tomography to demonstrate that this change in mechanical properties is not accompanied by structural differences between dry and wet sphere packings when compared at the same volume fraction. We characterize the structure of the packings by the average numbers of contacts of each sphere $⟨Z⟩$ and the shape isotropy $⟨{\beta }_0^{2,0}⟩$ of the Voronoi cells of the particles. Additionally, we show that the number of liquid bridges per sphere $⟨B⟩$ is approximately equal to $⟨Z⟩+2$, independent of the volume fraction of the packing. These findings will be helpful in guiding the development of both particle-based models and continuum mechanical descriptions of wet granular matter. (paper)