Nanorheology of bulk and interfaces via resonant thermal capillary fluctuations of bubbles

Resonant thermal capillary fluctuations (RTCFs) of bubble surfaces encode the mechanical response of fluid interfaces at the nanoscale, thereby providing direct access to their nanorheology. The presence of ions or surfactants is expected to modify the mechanical properties of the air/liquid interfaces in ways that are directly linked to bulk and interfacial rheology.

Sessile air bubbles were immersed in aqueous solutions of controlled composition, including pure water, salt solutions, and surfactant solutions. Thermal oscillations of the bubble surface were probed through the deflection of an atomic force microscope (AFM) cantilever gently contacting the bubble apex. The resulting power spectral densities exhibited multiple resonance peaks corresponding to different vibrational modes of the bubble. By fitting these peaks, mode-specific resonance frequencies and damping coefficients were extracted. Measurements were performed as functions of salt or surfactant concentration to quantify how solution composition alters interfacial mechanical properties and thereby determine the associated rheological parameters.

In salt solutions, the resonance frequency provides a direct measurement of the surface tension, while the damping coefficient determines the liquid viscosity. In surfactant solutions, the moduli obtained from damping coefficient measurements as a function of surfactant concentration agree with the expected Gibbs elasticity, a quantity generally inaccessible to conventional low-frequency techniques due to the dominant role of bulk surfactant diffusion. This methodology therefore establishes a high-resolution tool for investigating interfacial rheology at the nanoscale and at high frequencies.