Monitoring water meniscus formation at nanocontacts with shear-force acousto near-field microscopy

Shear-force acoustic near-field microscopy (SANM) is employed to monitor stochastic formation and post dynamic response of a water meniscus that bridges a tapered gold probe (undergoing lateral oscillations of a few nanometers amplitude at constant frequency) and a flat (gold or silicon oxide) substrate. As the probe further approaches the substrate, its amplitude decreases. Shear forces (of yet unknown precise origin) are typically invoked to explain the apparently pure damping effects affecting the probe's motion. Herein, SANM measurements underscore instead the role of near-field acoustic emission from the water meniscus as an elastic energy dissipation channel involved in shear interactions. A simplified thermodynamic argument is provided to justify the formation of a water meniscus between the probe and the sample once they are at sufficient separation distance. The reported measurements focus on the role played by the tip's geometry (by using probes of slender and chubby apex termination). The results shed some light on the potential origin of the so-called shear forces, invoked in many scanning probe microscopy applications, but not yet well understood.