The Pull-Off Force and the Work of Adhesion: New Challenges at the Nanoscale

The pull-off force required to separate two surfaces has become a convenient metric for characterizing adhesion at the micro- and nanoscales using cantilever-based force sensors, such as an atomic force microscope (AFM), e.g., as a way to predict adhesion between materials used in MEMS/NEMS. Interfacial Force Microscopy (IFM) provides unique insight into this method, because its self-balancing force-feedback sensor avoids the snap-out instability of compliant sensors, and can estimate both the work of adhesion and the pull-off force that would be measured using a compliant cantilever. Here, IFM is used to illustrate the challenges of determining the work of adhesion from the pull-off force in a nanoscale geometry. Specifically, adhesion is evaluated between a conical, diamond indenter and three surfaces relevant to MEMS/NEMS adhesion problems: silicon, a model insulator and a compliant polymer surface.