From single particle AFM studies of adhesion and friction to bulk flow: Forging the links

The atomic force microscope (AFM) has been used to study inter-particle contacts in air for a range of model particles and cohesive granular materials of commercial importance. Adhesion (or pull-off force), friction and its load dependence, and particle size, morphology and roughness were measured for glass ballotini, fumed silica, alumina, limestone, titania and zeolite. Particle-wall contacts and effects of relative humidity were also studied. Most of the results, after allowing for roughness, are consistent with JKR contact mechanics and capillary bridge theory; however, the main object of the present work is to demonstrate semi-quantitative links between the AFM measurements and related bulk flow and cohesion measurements performed in parallel on the same materials. A simple model of a particle assembly will be used to compare average contact forces in typical single-particle AFM experiments and typical bulk experiments, and thus identify those regimes of powder flow where the two approaches overlap, and AFM measurements may be used with some confidence in more sophisticated modeling based on distinct element analysis (DEA). Four areas will be discussed briefly: (1) The apparent analogy between bulk yield loci and single-particle friction-load data; (2) Cohesion data and particle size effects; (3) Bulk tensile strength and single particle pull-off force; (4) Bulk wall friction and single-particle-wall friction. It is found that typical single-particle AFM experiments and bulk shear experiments converge for small particles (similar to4 mum) and low consolidation stress, when the average inter-particle contact forces are of the order 20-100nN, involve single or few asperities, and are not much larger than pull-off forces. For large particles and high consolidation loads the data do not overlap and AFM measurements may be less useful as input to simulations where sliding friction is less important, and where large normal contact forces dominate over tangential forces and are responsible for the shear strength.