High-Frequency Contact Mechanics of Sphere-Plate Contacts: Fundamentals and Applications in Acoustic Sensing

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The project is concerned with the properties of sphere-plate contacts under high-frequency tangential loads. Important parameters to be studied are the contact stiffness and the energy dissipated per cycle. Both are a function of the oscillation amplitude. In terms of instrumentation, the proposed work builds on the shifts of the resonance frequency and of the resonance bandwidth of quartz crystal resonators, which are brought into contact with the spheres of interest. Nonlinear behavior shall be quantified making use on third-harmonic generation. Also, an evolution of the contact properties on a time scale slower than the frequency of oscillation shall be accessed with a fast mode of measurements, which has a temporal resolution of about 1 millisecond. Experiments shall occur both in air and in liquids. Of special interest are contacts established across a soft coating (for instance composed of a polymer brush or a supported liquid bilayer). Side aspects are the control and the variation of the electric potential of the resonator surface and the measurement of steady normal forces. A central question is, whether the transition from a sticking to a sliding contact (which is observed at MHz frequencies, as well) should be portrayed as an instability on the time scale of the oscillation. Alternatively, the two limiting states (sticking and sliding) might both be states obeying linear response, where the sticking state would be predominantly elastic in nature, while the sliding state would be predominantly viscous. In the latter scenario, the contact's structure and geometry will evolve over time towards the liquid-like state after the oscillation has been turned on. This evolution shall be kinetically resolved with a fast mode of measurement. If a slow (slow compared to the MHz timescale) evolution can be observed, the mechanisms governing the evolution shall be studied. Further, it shall be investigated, under which conditions this evolution progresses into the direction of lowered dissipation per cycle ("shake-down"). Another phenomenon to be studied with regard to underlying mechanisms is the occasionally observed increase of contact stiffness at high amplitudes (possibly connected to "junction growth"). The research shall have technical applicants in the field of acoustic sensing. A methodology shall be developed, by which soft layers adsorbed to the surface of a quartz resonator turn into interlayers of sphere-plate contacts. The layer's mechanical properties (linear and nonlinear) shall become accessible in this way, which shall lead to a more profound understanding of such soft systems.