Doo-In Kima, Jaroslaw Grobelnya,b, Namboodiri Pradeepa and Robert F. Cooka

a. National Institute of Standards and Technology, Gaithersburg, MD 20899 USA
b. University of Lodz, 90-236, Lodz, Poland



Interfacial mechanics at the nanoscale is greatly influenced by the humidity of the environment due to spontaneous condensation and the formation of a water meniscus between contacting surfaces. The force associated with the phenomenon called the “meniscus force”, modifies the contact geometry altering the adhesion mechanism between the surfaces. The origin of adhesion at nanoscale contacts in humid air is investigated by pull-off force measurements using atomic force microscopes in controlled environments from ultra-high vacuum through various humidity conditions to water. An equivalent work of adhesion (WOA) model with a simplified interface stress distribution is developed, combining the effects of screened van der Waals and meniscus forces, that describe adhesion in humid air and which self-consistently treats the contact stress and deformation. Although the pull-off force is found to vary significantly with humidity, the equivalent WOA is found to be invariant. Increasing humidity alters the nature of the surface adhesion from a compliant contact with a localized, intense meniscus force to a stiff contact with an extended, weak meniscus force.