  We investigate the wicking in granular media by considering layers of grains
at the surface of a liquid, and discuss the critical contact angle below which
spontaneous impregnation takes place. This angle is found to be on the order of
55{\deg} for monodisperse layers, significantly smaller than 90{\deg}, the
threshold value for penetrating assemblies of tubes: owing to geometry,
impregnating grains is more demanding than impregnating tubes. We also consider
the additional effects of polydispersity and pressure on this wetting
transition and discuss the corresponding shift observed on the critical contact
angle.
