Dry and wet nano-scale impact dynamics of rough surfaces with or without a self-assembled monolayer

In nanoscale conjunctions, molecular interactions cannot be ignored, nor surface energy effects, adhesion of surface asperities, or their stiction by any wetting action of an intervening fluidic media. Many experimental micro-electromechanical systems (MEMS) are prone to failure or malfunction due to structural degradation (wear and damage) of their load-bearing and power-transmitting conjunctions. Remedial solutions have been attempted by surface treatments, such as the introduction of self-assembled monolayers (SAMs) that adhere to the surfaces, and which are hydrophobic in nature. The physics of many of the contributing phenomena, such as adhesion, meniscus action, or electrostatics, are reasonably well understood, but their interactions under small-scale impacting condition is less investigated. The problem has a transient nature, owing to inertial dynamics, surface topography, attractive surface energy of asperities, and formation of any condensates. The interplay between these kinetics, using the established models, can be quite complex, particularly with regard to prediction of contact area and pull-off force in rebound of impacting pairs. The behaviour can vary from near classical Hertzian to that dominated by adhesion. The repercussions can be very significant in power transmission or load bearing for these small devices. This paper attempts to contribute to the growing understanding of contact behaviour in smallscale.