Shear Waves Do Not Propagate in Liquids

When discussing the benefits of acoustic waves close to surfaces, it is essential to not confuse MHz shear waves with conventional ultrasound. Conventional ultrasound employs compressional (longitudinal) waves. Compressional waves do propagate in liquids. For that reason, they may be reflected from the opposing wall of the cell, which complicates the experiment. Shear waves in liquids decay within less than a micron, and the shear-wave resonator therefore only sees the portion of the sample immediately adjacent to the surface (Fig. 8.2). For a 5 MHz bulk acoustic wave (BAW) resonator in water, the penetration depth of the wave, 8, is 250 nm. Admittedly, even shear-wave resonators sometimes do emit a small component of compressional waves. These are sources of artifacts. 

The compressional component is caused by the nonideal shape of the resonator (energy trapping, in particular). 

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