Monitoring near-field acoustic emission from confined water under shear interaction with blunt tips

Abstract

Mesoscopic water adhered to a surface at ambient conditions is typically reported to behave as sticky glue. Still, Shear-force Acoustic Near-field Microscopy (SANM), complemented with the Whispering Gallery Acoustic Sensing (WGAS) technique, has demonstrated the ability of a water meniscus, formed at the nanometer-sized gap between a sharp probe and a flat surface, to remain flexible enough for emitting near-field acoustic waves while being subjected to shear interactions exerted by a laterally oscillating probe. To gain insights on the meniscus formation process, as well as to better understand its viscoelastic response, purposely blunt probes (composed of multiple sharp asperities) are used. The experimental results suggest the stochastic formation and break of water bridges at multiple asperities on the probe, each providing an energy dissipation channel from the lateral motion of the probe to the fluid. Despite heavily damping the lateral motion of the probe, the ability of the large fluid meniscus to emit acoustic wave is recovered by properly increasing the amplitude of the lateral oscillations of the probe. Overall, monitoring the behavior of large meniscus adhered to a blunt probe allows to infer the behavior of smaller volume meniscus adhered to sharp probes.