The Effect of Grain Size and Shape on Sliding Friction of Wet Granular Media

Abstract The wet granular material creates networks in which interstitial liquid provides capillary bridges needed to hold grains together. There is an optimal fraction of the interstitial liquid in which the bridges are formed and the friction coefficient is minimal. We found that the size of the grains affects the friction of wet granular media. Our observations demonstrates that the optimum volume fraction, in which the friction coefficient is minimized, increases with grain size, but for sand immersed in water, this minimum friction coefficient itself increases with size and for glass beads immersed in silicone oil it decreases, indicating that the shape of the grains also has an effect on this friction. It is also shown that there is a crossover point for grain size at which the network effect created by capillary bridges is dominated by wet granular media. This crossover point is found.