Critical phenomena of water bridges in nanoasperity contacts

This article discusses capillary forces measured by scanning force microsco py (SFM), which, as recently reported, show a discontinuous behavior at a l ow relative humidity between 20% and 40% depending on the solid surfaces. A capillary force discontinuity is very interesting in terms of a possible p hase change or restructuring transition of bulk water in the interfacial so lid-liquid region. Unfortunately, we have found that SFM measurements show an inherent weakness in the determination of the origin of the forces that are obtained during pull-off measurements. This article critically discusse s the origin of the adhesive interactions as a function of relative humidit y with chemically modified probing surfaces. Our measurements indicate that force discontinuities in pull-off measurements are strongly affected by th e inability of the liquid to form capillary necks below a critical threshol d in relative humidity. In the course of this article, we will discuss roug hness effects on capillary forces and provide a modified capillary force eq uation for asperity nanocontacts. (C) 2001 American Institute of Physics.