Ion dynamics and resonance in an electrical double layer undergoing periodic excitation by a surface acoustic wave

Electrical double layers (EDLs) of ions are a surface phenomenon. They appear at the interface between charged surfaces and electrolyte solutions and are one of the most fundamental and abundant mechanisms in nature. They give rise to the complexity of biology and to life as well as to countless industrial processes and products from water desalination to shampoo.
Steady and quasi-steady measurements of EDL properties are commonplace.  However, the EDL Debye length, measured in nanometers, and the intrinsic ion relaxation time through the EDL’s Debye length, measured in nanoseconds, have made measurements of ion dynamics inside isolated transient EDLs elusive. A previous theory [1] suggests that dynamic measurements of EDLs are possible by using a high frequency mechanical excitation.
We show in expeirment a strategy to capture EDL ion dynamics by imposing a field effect––an evanescent mechanical wave––in an electrolyte solution, which is generated by a MHz-frequency surface acoustic wave (SAW) in a neighboring substrate; the evanescent wave appears at similar length and time scales to the ones of ion motion in the EDL. The EDL and evanescent wave—existing at the same space and time—become entangled. Ions in the EDL diffusive layer undergo dynamic mechanical vibrations at the SAW frequency and support a same frequency leakage of AC (alternating) electrical field off the EDL that we measure. Moreover, a SAW excitation frequency comparable to the inverse of an ion relaxation time supports electro-mechanical ion resonance. The latter maximizes the ion mechanical vibration and the magnitude of the electrical field leakage, rendering ion-specific relaxation times measurable and giving the intrinsic rate of EDL charge and discharge by specific ions. [2]

Figure 1. Positive (+) and negative (–) ions vibrate in an electrical double layer (EDL), which emanates from a solid substrate that supports a MHz-frequency traveling surface acoustic wave (SAW). The SAW dynamically perturbs ion positions, resulting in the leakage of an electrical field off the near-equilibrium EDL, where red and blue indicate fast and slow advective ion motion within the evanescent SAW viscous penetration length in the solution [3].
[1] O. Dubrovski and O. Manor, Langmuir, 2021, 37,14679–14687
[2] S. Aremanda and O. Manor, J. Phys. Chem.C, 2023, 127, 20911–20918
[3] O. Manor, L.Y. Yeo, and J.R. Friend, J. Fluid Mech., 2012, 707, 482–495