The study, published in Frontiers in Acoustics, builds on a previous experiment in which the researchers excited EDLs directly on the piezoelectric lithium niobate substrate of a surface acoustic wave (SAW) actuator. In that setup, the EDL was exposed to two influences at once. The Rayleigh-type SAW itself is a mechanical vibration traveling near the surface of the solid, and it drags with it a sub-micrometer-thick evanescent boundary layer flow in the neighboring water. But in a piezoelectric substrate, the same wave is accompanied by a voltage signal counterpart that spreads an electric field into the electrolyte, periodically migrating ions in its own right. Which of the two dominated the excitation of the EDL remained unclear. The periodic AC voltage applied to the actuator governs both the SAW intensity and the level of its electrical potential counterpart, so the two effects were convoluted in every measurement conducted directly on the actuator.

To disentangle them, the team transferred the SAW from the actuator into an ordinary glass slab. Glass is a dielectric with no mobile charges, so it supports pure mechanical vibration while attenuating the electrical field emanating from the piezoelectric substrate. The integrated device consisted of a 1-mm-thick glass microscope slide attached to a flat lithium niobate substrate of a 20 MHz SAW actuator, with a thin film of glycerol serving as an ultrasonic impedance-matching liquid between the two solids. The glycerol spread into the gap by capillary action, trapping itself there and coupling the mechanical power from the actuator into the glass. At the far end of the glass, a strip of wet paper absorbed much of the wave to suppress reflections.

The measurements revealed something unexpected about how the mechanical field travels through this sandwich. The normal vibration amplitude on the glass surface was similar to that on the lithium niobate substrate, and both supported a wavelength of roughly 200 micrometers. Near the interdigital transducer—the IDT that generates the wave—the vibration on both solids behaved as a nearly propagating wave. Farther away, at separations of 8 and 12 millimeters, it became a combination of propagating and standing waves, likely because the vibration source sits on one side of the device while reflections return from the opposite edge. Intriguingly, the SAW did not attenuate by simple diffraction into the glass, as it would if the glass were a half-space; instead, the vibrations in the two substrates appeared intimately connected, with power flowing back and forth between them. The behavior suggests the vibration in the glass is akin to a Lamb wave, spread throughout the slab, continuously absorbing and reflecting power to and from the actuator.

Meanwhile, the electrical field accompanying the wave in the piezoelectric substrate decayed exponentially through the glass. Solving the Poisson equation for the potential in a half-space of glass, subject to the spatiotemporal potential signal at the lithium niobate surface, showed that the field reaches a negligible magnitude at the upper surface of the glass slab. The measured electrical potential there was three to four orders of magnitude smaller than the input potential to the actuator. In other words, the mechanical wave alone is what reaches the upper glass surface to excite the EDL at the glass/electrolyte interface—precisely the isolation the experiment required.

The glass surface itself becomes spontaneously charged at levels of tens of millivolts when in contact with an electrolyte, supporting an EDL much like the one on the piezoelectric substrate. The researchers dispensed sessile drops of two strong electrolytes—sodium nitrate and potassium chloride—chosen because their small ion pairs keep the double layers near ideal, well described by the Boltzmann–Poisson equation at low ionic concentrations. Sodium nitrate offers ions with dissimilar diffusion coefficients, while potassium chloride has ions with nearly identical ones at the precision of the measurement. A working microelectrode was positioned just 30 to 40 micrometers above the glass surface—closer than the 200-micrometer SAW wavelength, to avoid dispersion of the leakage field—with a reference electrode about a millimeter away in the bulk of the drop. An RF lock-in amplifier measured the 20 MHz component of the electrical leakage, the linear same-frequency response of the EDL to the exciting vibration.

What the electrodes recorded was a spectrum of electrical field leakage from an out-of-equilibrium EDL. At equilibrium, the nanometer-thick ion cloud fully screens the field of the surface charge. But the megahertz vibration renders the EDL out of equilibrium, and it leaks an electrical field into the electrolyte bulk—a field proportional to the zeta potential of the surface and modulated in time by the motion of the ions. The leakage takes on the frequency of the exciting wave and its harmonics. Crucially, the spectra were non-monotonic: they took local maxima where the period of the mechanical vibration synchronized with the relaxation times of specific ions in the EDL. At those frequencies, the synchronization maximizes the periodic displacement of the ions, amplifying the field leakage. The peaks are specific to ion mobilities, effectively reading out an array of ion relaxation times—and hence mobilities—within the double layer.

Three points of comparison with the previous study, in which the EDL was excited directly on the piezoelectric substrate, confirmed that the mechanical component alone reproduces the full phenomenon. First, the measured voltage peaks appeared where the vibration period matched the ion relaxation time in both studies, with the peaks for sodium nitrate and potassium chloride arising at acoustic-to-ion relaxation time ratios roughly 15 percent smaller than theoretical values, while their harmonics came closer to theory; for potassium chloride, the measured peaks agreed well with the theoretical values. Second, the magnitude of the leakage was of the same order in both studies, though the ratio of measured to applied voltage was greater in the previous study by a factor of about two to three—attributable to a lower vibration displacement on the glass surface per applied voltage level. Third, scaling the measured voltage by the applied voltage collapsed the data onto unifying curves in both cases: one curve for sodium nitrate, and two curves for potassium chloride, the second associated with a response about 15 to 20 percent greater. The similar collapse suggests similar governing physical mechanisms, with the double curve in the potassium chloride case flagged as a caveat for further study.

The work is reminiscent of classic electroacoustic measurements, in which bulk ultrasonic waves interacting with ions in electrolytes or charged colloids generate a potential difference across an entire volume of dispersion—techniques long used to analyze ionic species and, most notably, the zeta potential of particle suspensions. The difference here is that the ultrasonic vibration is generated in the solid itself, exciting ions in an isolated EDL emanating from a single flat surface. That opens the door to measuring the static and dynamic electrokinetic properties of flat solid surfaces of many kinds, provided they support a corresponding mechanical vibration. Silica-based glass, the platform used in this experiment, is a workhorse of surface chemistry manipulation, making it a versatile stage for studying how different surface chemistries shape ion dynamics in EDLs—relevant to membranes, batteries, and supercapacitors.

There is also a cautionary message embedded in the findings for the growing field of SAW-actuated micro- and nanofluidics. If a mechanical vibration traveling along a solid in an electrolyte vibrates the ions of the EDL at its own frequency, then double layers on such platforms are not at equilibrium—and their electrical properties may differ substantially from equilibrium ones. Out-of-equilibrium EDLs govern ion transport through ion-selective membranes and the stability of liquid films of electrolyte solutions, and they may alter membrane ion-screening properties and film stability in ways that equilibrium theory cannot predict. For anyone designing acoustic microfluidic devices, the hidden nanometer-thin layer at every solid/liquid interface may be far more dynamic than it appears.

Subject of Research: Acoustofluidics

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

Article References: Aremanda, S., Li, Y., Onuh, G., & Manor, O. (2025). Measurement of dynamic electrokinetic effects at the glass/electrolyte interface using a mega-Hertz-level mechanical wave. Frontiers in Acoustics, 3, Article 1568083. https://doi.org/10.3389/facou.2025.1568083

Image Credits: AI Generated

DOI: 10.3389/facou.2025.1568083

Keywords: surface acoustic wave, electrical double layer, electrokinetics, electroacoustics, acoustofluidics, ions, microfluidics, Measurement, dynamic, electrokinetic, effects, glass