This study investigates the fundamental frictional behavior of granules through experimental analysis under two direct-shear testing scenarios: grains-assembly shearing and grains-to-surface shearing (when the shear plane is between the grains and a flat solid surface), incorporating new experimental data with comparison to previous findings. By varying grain mineralogy, morphology, and pore liquids, we identify key differences between the two systems. The results show that grain-assembly friction is influenced by grain morphology but not by mineralogy, whereas grains-to-surface friction exhibits the opposite trend. The presence of pore liquid also has contrasting effects: it reduces friction in grain-assemblies due to lubrication but increases friction of grains-surface systems due to solid-liquid adhesion. This paper explains these trends by hypothesizing a link between each shearing scenario to distinct grain displacement mechanisms—particle sliding or rolling (rearrangement). It also shows that the friction of an 'ideal' granular system, made of uniform, rigid, and smooth spheres, sets a threshold for the effects of lubrication and adhesion.
Miron et al. (Tue,) studied this question.
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