Acousto characterization of fluid-like mesoscopic films under shear

Full understanding of the physics underlying the striking changes-in viscoelasticity, relaxation time, and phase transitions-that mesoscopic fluid-like systems undergo when placed under confinement or when adsorbed at solid surfaces constitutes a long standing scientific challenge. One of the methods used to characterize these films consists of bringing a solid boundary closer to another solid boundary (while in relative lateral periodic motion) with a liquid trapped in between. In addition, using a tapered probe (~ 50 nm apex diameter) as one of the boundaries improves the lateral resolution of the measurement. In this scenario, the dynamics of the fluid is inferred from the changes in the tapered probe´s motion. However, due to the complexity of the film´s dynamics, different and sometimes conflicting experimental results are reported; in particular, for example, whether the motion of the probe changes due to its interaction with the fluid alone, or due to its intermittent mechanical contact with the solid substrate. Newer analytical methods would be highly desirable. Herein we report the monitoring of mesoscopic film dynamics from an acoustic measurements perspective (complemented with other more conventional sensing methods for control and comparison purposes). More specifically, two acoustic-based methods, Whispering-Gallery Acoustic Sensing or WGAS (that uses an acoustic sensor attached to a tapered probe) and Shear-force/Acoustic Near-field Microscopy or SANM (that uses an acoustic sensor attached to the solid substrate), monitor the effects that shear-force interactions exert not only on the laterally oscillating probe but also on the trapped mesoscopic fluid itself (as acoustic waves engendered at the fluid film couple into the static substrate and subsequently reaching the SANM acoustic transducer). One significant result of these measurements constitute the supporting evidence that the probe´s motion is affected even when not in mechanical contact wit- the solid substrate, hence highlighting the role played by the adsorbed mesoscopic fluid layer as the source of the shear force interactions. On the other hand, to further support the SANM working principle (i. e. the measurement of acoustic waves engendered at adsorbed films of nanometer-sized) control experiments have also been performed for interrogating the dynamics of small millimeter-sized drops of water.