Currently, the equilibrium forces between ideal molecularly smooth) surfaces in inert vapors are reasonably well-understood. But for non-spherical, polydisperse, and particles whose surfaces are not smooth even on the nano-scale, fundamental understanding is still rudimentary. In addition, flow involves changes in both the normal (e.g., adhesion) and lateral (e.g., shear or friction) forces between particles, which move in a complex 3D trajectory relative to each other. The situation becomes further complicated for interactions in condensable vapors, e.g., in humid air or air containing condensable organic molecules, where capillary condensation (liquid bridges) and capillary forces are now also involved.
More complex still, for particles that are not perfectly rigid or elastic, but viscoelastic or ‘anelastic’, slow deformations and rearrangements can occur over time due to the high local stresses (both compressive and tensile) on these particles, leading to ‘dynamic aging effects’ such as slow deformations and creeping flow, and various types of instabilities (in addition to the normal type of sudden slip instability). The aim of this review or report is to discuss these outstanding issues in terms of the equilibrium and dynamic (non-equilibrium or history-dependent) forces between particles, and what further experiments and theoretical modeling need to be done to enable us to control these forces and predict particle (powder, granular) flow. Thus, where appropriate, each section ends with a paragraph entitled Outstanding issues and challenges describing the writer’s assessment of the important unresolved issues discussed in that section. At the very least, it is hoped that the various experiments and modeling strategies proposed will allow for scaling up of laboratory experiments to predict behavior under field conditions. This calls for ‘scaling theories’ or equations giving the critical conditions, e.g., to flow, in terms of the various length scales involved (particle size, rms roughness, shape anisotropy, Kelvin radius determined by the relative humidity, height of sample, etc.) and other relevant parameters (particle composition, elastic properties, extraneous vibrations, previous history of the sample, temperature, etc.). The focus will therefore be on interparticle interactions in typical atmospheric conditions. Colloidal interactions (in solution) will not be covered except in cases where particles are separated by thin layers of liquids capillary-condensed from undersaturated vapors.