Interactions between electrical double layers and MHz-frequency surface acoustic waves: Revisiting the electroacoustic phenomenon near a planar substrate*

The electrical double layer of ions (EDL) to appear next to charged surfaces in electrolyte solutions is fundamental across countless natural and artificial systems and may considerably vary in properties between one system to another. Its applications range from supporting electrophoresis and electrowetting to governing the kinetics of particulate coagulation and from governing the folding structures of proteins and the surface properties of membranes to determining the rate of particulate adsorption onto a substrate. However, the generic nature of the EDL and the interplay between intimate physical mechanisms therein is still an enigma even now, more than a century since it was discovered. A main difficulty in investigating dynamic EDL effects is the short EDL length scale of usually several nanometers and the EDL relaxation time of nano- to micro-seconds. We employ a MHz-frequency mechanical vibration near the surface of a solid substrate, i.e., a Rayleigh type surface acoustic wave (SAW), to excite the EDL within its length and time scales and hence obtain dynamical information about the physics therein.

    We use SAW to excite the EDL at the interface between the solid and electrolyte solution. The excitation renders a partial electrical discharge in the EDL and dynamic variations in ion density therein, which support the leakage of a measurable electrical signal off the EDL and into the electrolyte solution. The leakage possesses a frequency spectrum which corresponds to an array of mechanical-electrical resonance effects in the EDL. We show that the measurement agrees to leading order with a dedicated linear theory for the near equilibrium distortion of the EDL. The agreement facilitates a mean for deciphering the measured signal to potentially give new and direct insights about the EDL and its subcomponents.