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Understanding the principles governing the striking properties that mesoscopic fluids display under confinement (namely, enhanced shear viscosity, prolonged relaxation time, confinement-induced phase transformation, hydrophobic effects beyond commonly accepted molecular interaction ranges) remain major challenges in condensed matter physics. To study these complex fluids we are using Shear-force/Acoustic Near-field Microscope (SANM) and Whispering Gallery Acoustic Sensing (WGAS), recently introduced by our group. In particular, we investigate the role played by water films trapped between two hydrophobic/philic solid boundaries under relative oscillatory motion. By using the apex of a tapered probe as one of the confining solid surfaces, while having the unprecedented capability to monitor the near-field acoustic emission from the trapped mesoscopic fluid, SANM constitutes an alternative tool to study surface wetting properties with nanometer lateral resolution.
We present the genealogy of SANM and WGAS as the outgrowth of other SPM techniques used to characterize and fabricate nanostructures, where such fluids play an important role (for implementing probe-sample distance control) and may be the source of unexpected and/or non-reproducible results. We present results from systematic measurements carried out at different experimental conditions, namely tip geometry, environmental humidity, and sample cleaning procedures.
In the final part we outline a provocative hypothesis. We focus on the role of the fluid’s acoustic emission as an elastic energy dissipation channel involved in shear interactions. The changes in the SANM acoustic signal are explained in terms of variations in the (constant volume) water-bridge geometry during its compression, which causes a corresponding change in the Laplace pressure; the smaller radius of curvature, the larger the pressure difference, the stronger the sound intensity. These hypothesis, backed up by acoustic experiments, shed light on the potential origin of the so called “shear-force mechanism”, invoked in many scanning probe microscopy applications, but not yet well understood.