PARTICLE ADHESION AT THE NANOSCALE

This article attempts to connect macroscopic observations of particle adhesion with the known interatomic forces which bind particulate interfaces together by studying contact between a plane surface and a sphere of smaller and smaller diameter. The fracture of a contact between a plane and a macroscopic sphere depends on the nonuniform stress distribution across the contact spot, causing atomic attraction at the edges of the contact region. Interface atoms some distance inside the contact region do not contribute to the adhesion. In fact, these inner atoms are in compression and are pushing the particles apart rather than causing adhesion. When a smaller sphere adheres to a plane at the nanoscale, this nonuniform stress distribution cannot be possible and the stress across the contact must be more even. To prove this hypothesis, molecular dynamics (MD) simulations have been carried out to study the fracture behaviour of subnano sodium chloride crystals. The MD models show clean fracture across the contact junction, in agreement with the macroscopic fracture studies. The models included explicit interatomic potentials to calculate the adhesion forces and contact stress distributions during particle pulloff as sodium chloride particles were altered in size. The results show that there is stress concentration at the contact edge for the smallest particles with 16 atoms (4 × 4) in contact.