Turning Porous Functional Materials into Directional Transport Platforms with Unidirectional Surface Acoustic Waves

TL;DR

This paper demonstrates how unidirectional surface acoustic waves generated by floating-electrode transducers can actively induce and control high-velocity, directional flow in porous materials, surpassing diffusion and capillary-driven flows.

Contribution

It introduces FEUDTs as a novel method to convert porous media into actively pumped platforms using unidirectional surface acoustic waves, with practical design rules based on pore size and wave wavelength.

Findings

Flow velocities up to 0.6 mm/s achieved at sub-watt power

Transport efficiency peaks when SAW wavelength matches pore size

FEUDTs outperform diffusion and capillary-driven flow in porous media

Abstract

Porous media underpin absorption, filtration, separation, and high-area interfacial transport in chemical and diagnostic systems, yet sustained directional flow through them remains difficult because tortuous pore networks and strong acoustic losses promote bypassing, weak flow, and counterflow. Here, we show that floating-electrode unidirectional transducers (FEUDTs) convert porous materials into actively pumped transport platforms by generating predominantly unidirectional surface acoustic waves (SAWs) that couple more effectively than conventional interdigital transducers across wet multilayer interfaces. By varying pore size, permeability, sample thickness, and fluid viscosity, we find that transport is strongly enhanced when the SAW wavelength is comparable to the characteristic pore dimension, providing a practical design rule for acoustically activated porous media. Under these conditions, FEUDTs drive directional flow velocities up to 0.6 mm s$^{-1}$ at sub-watt input power, about 600 times faster than diffusion alone. FEUDTs also sustain pumping in prewetted porous media, where capillary contributions are removed, yielding velocities that exceed capillary-driven flow under matched conditions while remaining far above thermally induced transport. A reduced theoretical framework captures the main experimental trends and identifies transducer architecture, pore geometry, and actuation strength as the key parameters governing long-range, tunable transport in porous functional materials.