Review of Adhesion Fundamentals for Micron-Scale Particles

The effects of various fundamental forces on the adhesion of fine dust particles are reviewed. The particle-size and distance variation of surface-energy related (e. g., van der Waals) forces are compared to similar relations for static-electric image-forces for tribo-charged particles near (or contacting) conducting surfaces. The van der Waals force (between macroscopic spheres), a patch-charge image-force and static-electric image-forces all exhibit an inverse square variation with distance; however, these forces have dramatically different ranges-of-effect. The very short-range nature of van der Waals forces (of order 10nm) is a major reason that most real contacts, involving non-smooth surfaces, exhibit adhesion forces that are substantially lower than values predicted for smooth particles. Based on studies of Lunar and Martian regolith stimulant powders, triboelectric charges on fine particles appear to scale linearly with particle size. It is shown that below some threshold size, the adhesion (to conducting surfaces) of charged dust particles possessing such a linear charge-to-size scaling relationship, may be dominated by image-charge forces, instead of surface-energy related interactions. This is counter to what might have been expected from a cursory examination of the fundamental force relations, which would suggest van der Waals adhesion forces would dominate for small particle sizes.