Measurements of interfacial friction using quartz crystal resonators

We report on the interaction between quartz resonators and spheres approaching one of their surfaces. As firm contact is established, the resonance frequency increases, indicating an increased stiffness of the quartz-sphere system. When contact is just barely reached, additional dissipative processes are present as evidenced by an increased bandwidth. We also monitored the motion of the quartz surface in real time with a digital oscilloscope. By Fourier transformation of the oscilloscope trace the nonlinear components of the motion were determined. In contact, one observes an increase in the second harmonic amplitude which, for reasons of symmetry, is associated with periodic normal forces pushing the sphere away from the quartz plate