Mode-Resolved Mechanical Signatures of Light-Induced Dynamics at Nanoscale Aqueous Interfaces

We report mode-resolved mechanical evidence that visible-light illumination reversibly modifies the coupled conservative and dissipative response of a nanoscale tip-aqueous junction. Using the passive multimode resonance spectrum of an undriven atomic force microscope cantilever as a local mechanical readout, we observe reversible, near-field-localized changes in resonance frequency and line width upon 532 nm illumination of a deliquesced CaCl₂ droplet. Distance-dependent measurements, together with dry-glass and silicone-oil controls, confirm the signal is localized to the aqueous interface's near-field. Analysis of the two lowest eigenmodes rules out purely mass-loaded or conservative origins. Only a model considering both conservative and dissipative contributions reproduces all four modal observables. The extracted conservative perturbation matches a nanoscale capillary force-gradient estimate. The relaxation time scale is consistent with interfacial meniscus kinetics rather than local thermal diffusion. These results establish a nanoscale mechanical framework for probing illuminated aqueous interfaces, constraining the magnitude, dissipative character, and time scale for any proposed mechanism.