Figure 20.

An example of a film resonance. At some point, the frequency jumps, as discussed in the caption to Figure 19. Reprinted with permission from [96]. Copyright 2017 American Chemical Society. These experiments occurred in liquid. Film resonances can occur in liquids, as well, because Equation (49) also contains terms of the form $\tan \left({\tilde{k}}_{\mathrm{f}}{d}_{\mathrm{f}}\right)$.