Measurement of dynamic electrokinetic effects at the glass/electrolyte interface using a mega-Hertz-level mechanical wave

We measure the dynamic electrical properties of a spontaneously charged (electrified) glass surface in an electrolyte solution by using a MHz-level surface acoustic wave (SAW) actuator to introduce a same-frequency mechanical wave into the glass substrate. The mechanical wave vibrates ions in the nanometer-thick electrical double layer (EDL) to appear at the glass/electrolyte interface. The out-of-equilibrium EDL leaks an electrical field, which is modulated by ion vibration frequency to reveal the presence of ions and their dynamic motion. Adapting chirped interdigital transducer SAW actuator, we scan over multiple frequencies and depict an electrical field leakage spectrum that gives the dynamic relaxation times of ions in the EDL. In more detail, the excited EDL leaks an electrical field that reaches a local maximum when the SAW frequency matches the intrinsic frequency of an ion in the EDL (inverse of the ion relaxation time). We study wide range of electrolytes and combinations thereof and observe relaxation times of specific ions and of the compound motion of different ions of opposing charge.