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

该项目关注高频切向载荷下球板接触的特性。要研究的重要参数是接触刚度和每个周期消耗的能量。两者都是振幅的函数。在仪器方面，所提出的工作建立在石英晶体谐振器的谐振频率和谐振带宽的偏移上，石英晶体谐振器与感兴趣的球体接触。应利用三次谐波产生对非线性行为进行量化。此外，应使用快速测量模式来访问接触特性在比振荡频率慢的时间尺度上的演变，该模式的时间分辨率约为1毫秒。实验应在空气和液体中进行。特别感兴趣的是在软涂层（例如由聚合物刷或支撑的液体双层组成）上建立的接触。侧面方面是谐振器表面的电势的控制和变化以及稳定法向力的测量。一个中心问题是，从粘附到滑动接触的转变（在MHz频率下也可以观察到）是否应该被描绘为振荡时间尺度上的不稳定性。可替代地，两个极限状态（粘附和滑动）可以都是服从线性响应的状态，其中粘附状态在本质上主要是弹性的，而滑动状态主要是粘性的。在后一种情况下，在振荡开启后，接触的结构和几何形状将随着时间的推移向类液体状态演变。这种演变将通过快速测量模式进行动力学解析。如果可以观察到缓慢（与MHz时标相比缓慢）演变，则应研究控制演变的机制。此外，还应研究在何种条件下，这种演变会朝着降低每循环耗散的方向发展（“安定”）。另一个现象是要研究的基础机制是偶尔观察到的接触刚度增加在高振幅（可能连接到“结生长”）。该研究应具有声学传感领域的技术申请人。应开发一种方法，通过该方法，吸附到石英谐振器表面的软层变成球-板接触的夹层。该层的机械性能（线性和非线性）将以这种方式变得可访问，这将导致对这种软系统的更深刻的理解。

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.