Abstract
There has been much interest in the past two decades to produce experimental force profiles characteristic of the interaction between nanoscale objects or a nanoscale object and a plane. Arguably, the advent of the atomic force microscope AFM was instrumental in driving such efforts because, in principle, force profiles could be recovered directly. Nevertheless, it has taken years before techniques have developed enough as to recover the attractive part of the force with relatively low noise and without missing information on critical ranges, particularly under ambient conditions where capillary interactions are believed to dominate. Thus a systematic study of the different profiles that may arise in such situations is still lacking. Here we employ the surfaces of CaF2, on which nanoscale water films form, to report on the range and force profiles that might originate by dynamic capillary interactions occurring between an AFM tip and nanoscale water patches. Three types of force profiles were observed under ambient conditions. One in which the force decay resembles the well-known inverse-square law typical of van der Waals interactions during the first 0.5–1 nm of decay, a second one in which the force decays almost linearly, in relatively good agreement with capillary force predicted by the constant chemical potential approximation, and a third one in which the attractive force is almost constant, i.e., forms a plateau, up to 3–4 nm above the surface when the formation of a capillary neck dominates the tip–sample interaction.
Highlights
The study of the forces and energies released when a nanometric tip and a surface are progressively brought into contact has driven much of the recent investigation in atomic force microscopy (AFM) and has allowed for the mapping of materials properties while scanning [1,2,3] besides finding optimal imaging conditions [4,5]
Force vs distance curves were collected in both contact mode AFM and reconstructed from dynamic AM-AFM experiments on CaF2 crystals exhibiting water patches on the surface
Results indicate that dissipative processes occur that involve the formation and the rupture of a capillary bridge. This can be inferred from the high hysteresis in the retracting portion of the static force curves, which spans an average distance of about 30 nm and exhibits an adhesion force of almost 6 nN compared to the approach path
Summary
The study of the forces and energies released when a nanometric tip and a surface are progressively brought into contact has driven much of the recent investigation in atomic force microscopy (AFM) and has allowed for the mapping of materials properties while scanning [1,2,3] besides finding optimal imaging conditions [4,5]. Contact AFM measurements, in which the force is determined from the static deflection of the cantilever during approach [15], can readily record the tip–sample interaction force and have been used extensively to characterize a variety of nanoscale materials, from soft biomaterials (vesicles, viruses) [16,17], to organic thin films [18,19,20,21] and self-assembled monolayers [22] in liquid and in air, especially at those short separations where breakthrough events and sample mechanical deformations occur In such experiments, the jump-to-contact instability [6,7] screens even strong (van der Waals, capillary) [13,23] interactions and prevented the experimental access to that region in force curves where attractive forces dominate. This instability has been especially observed when working in air and when soft cantilevers were employed to increase the sensitivity [6,7,15,24]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
