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

Area of Science:

• Nanoscale science
• Surface chemistry
• Physical chemistry

Background:

• Understanding interfacial phenomena is crucial for many chemical and physical processes.
• Characterizing the dynamic response of aqueous interfaces under external stimuli requires sensitive techniques.

Purpose of the Study:

• To investigate the reversible mechanical changes in nanoscale tip-aqueous junctions under visible light illumination.
• To establish a mechanical framework for probing illuminated aqueous interfaces.

Main Methods:

• Utilized atomic force microscopy (AFM) with passive multimode resonance spectrum analysis.
• Employed 532 nm visible light illumination on a deliquesced CaCl2 droplet.
• Conducted distance-dependent measurements and control experiments with dry-glass and silicone-oil.

Main Results:

• Observed reversible, near-field-localized changes in resonance frequency and line width upon illumination.
• Ruled out purely mass-loaded or conservative origins for the observed mechanical changes.
• A model incorporating both conservative and dissipative contributions successfully reproduced all four modal observables.

Conclusions:

• The study demonstrates light-induced reversible modifications in coupled conservative and dissipative responses of aqueous interfaces.
• Extracted conservative perturbation aligns with nanoscale capillary force-gradient estimates.
• The observed relaxation time scales suggest interfacial meniscus kinetics, not thermal diffusion, are dominant.