Friction at the grain-scale: the role of inter-particle friction in granular media and its influence on grain-scale bed behaviour

Granular media has near omnipresence in nature and is the second most processed substance in industry, after water. It is well accepted that it exhibits a wide spectrum of macro-scale behaviour which is ultimately determined by the grain-scale interactions of its constituent particles 123; but there is still much to be discovered about those grain-scale interactions themselves. Away from the free surface of an agitated granular bed, the dominant grain-scale interactions are relative sliding and rolling between neighbouring particles 4, and it is this sliding and rolling which is the subject of this research.In these experimental lab-based tests, dry and wet ideal granular beds are harmonically compressed via a moving side-wall and their responses captured via high-speed imaging. The granular media itself is a quasi-2D bed of polydisperse discs consisting of an even mixture of five different disc diameters ranging from 11mm to 36mm. The cyclic compressions are specifically designed to impose a jamming effect within the granular beds, before subsequent relaxation and deformation.Use of the photo-elastic technique provides a window through which the grain-scale behaviour of the beds can be examined, as networks of inter-particle contact forces, known as force chains, become visible. Disc rotation is measured by tracking lines drawn onto each disc, providing useful insight into the sliding and rolling inter-particle interactions at the grain-scale. First, the behaviour of a dry granular bed is examined, and then a thin layer of glycerol is spread onto the edges of each individual disc in order for the behaviour of an equivalent wet granular bed or at least, a bed with reduced inter-particle friction to be examined. The behaviour of these beds are then compared to one another, and the results used to discuss how changes to friction at the grain scale affects the behaviour of granular bodies.1 Singh, S., Murthy, T.G.: Evolution of structure of cohesive granular ensembles in compression. International Journal of Solids and Structures 238(1), 111359 (2022)2 Jiang, M., Yu, H., Harris, D.: A novel discrete model for granular material incorporating rolling resistance. Computers and Geotechnics 32(5), 340357 (2005)3 Oda, M., Konishi, J., Nemat-Nasser, S.: Experimental micromechanical evaluation of strength of granular materials: effects of particle rolling. Mechanics of Materials 1(4), 269283 (1982)4 Moss, J., Glovnea, R.: Behavioural responses to horizontal vibrations of quasi-2D ideal granular beds: an experimental approach. Granular Matter 25(4), 63 (2023).