The shear modulus of wet granular matter

The strength of different wet granular materials is investigated as a function of the liquid volume fraction by measuring the elastic shear modulus, G'. We show that the optimum strength is achieved at a very low liquid volume fraction of 1–3%. Surprisingly we find that the macroscopic strength of different wet granular materials depends with a power of 2/3 on the microscopic elastic modulus of the individual grains, with a power of - 1/3 on the radius, and with a power of 1/3 on the surface tension. This can be explained by assuming that the attractive capillary force between two grains deforms the grains elastically, yielding a "spring constant" for further deformation. Averaging over many grain-grain orientations allows us to predict the macroscopic shear modulus in excellent agreement with our experiments.