Chemical Reactivity of Confined Water under Surface Acoustic Wave Modulation.

Water is commonly regarded as a chemically inert medium in microfluidic systems, with chemical transformations attributed primarily to added reagents or catalysts. Here, we show that surface acoustic wave (SAW) actuation in a microfluidic channel establishes an electroacoustically modulated confined-water environment in which the intrinsic chemical reactivity of water is markedly enhanced. Focused SAWs impose a spatially heterogeneous physical field at the solid-liquid interface, characterized by localized electric fields distinct from static contact conditions. Within this environment, reactive oxygen species are continuously generated and hydrogen peroxide accumulates reproducibly at the parts-per-million level. Systematic variation of flow rate, dissolved oxygen availability, and acoustic driving strength constrains hydrogen peroxide formation to a water-centered, radical-mediated framework in which hydroxyl radicals play a central role, while molecular oxygen contributes in a condition-dependent manner. These findings demonstrate that electroacoustic modulation provides a physical route to accessing nonequilibrium chemical states of confined water without altering its composition.