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

Sudeepthi AremandaYifan LiGideon OnuhOfer Manor^*

• Technion Israel Institute of Technology, Haifa, Haifa, Israel

We measure the dynamic electrical properties of a spontaneously charged glass surface in an electrolyte solution by using a MHz-level surface acoustic wave (SAW) actuator to introduce a same frequency mechanical wave in 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 electrical field, which is modulated by ion vibration frequency, revealing the presence of ions and their dynamic motion. A previous study excited EDLs on the piezoelectric lithium niobate substrate of a SAW actuator in contact with an electrolyte solution; it remained unclear whether the mechanical or electrical components of the SAW in the piezoelectric substrate dominated the EDL excitation. We isolate the SAW mechanical component in glass and show that it introduces a similar ion electrokinetic vibration in the excited EDL at the glass/electrolyte interface using Sodium Nitrate and Sodium Chloride solutions as electrolytes:The measured electrical field leakage spectra are of similar magnitude to the ones measured in the previous study and exhibit similar non-monotonic behaviors, taking local maxima where the SAW period (inverse of its frequency) is synchronized with the ion relaxation times in the EDL. At these frequencies, the synchronization maximizes ion vibration displacement, thereby amplifying the electrical field leakage. Our findings may be used to study electrokinetic properties of solid surfaces and ion dynamics in EDLs. Moreover, SAW-actuated fluidic platforms may support out-of-equilibrium EDLs of consequence to, e.g., applications involving ion selective membranes and the film stability of electrolyte solutions.