Visco-capillary response of surfactant laden air-water interfaces measured by dynamic colloidal-probe AFM
Phys. Rev. Fluids 11, 024002 – Published 5 February, 2026
DOI: https://doi.org/10.1103/tqt7-hsz9
Abstract
Understanding the rheological properties of the air-water interface is critical for applications across soft matter, biological systems, and microfluidics. In this study, we used dynamic colloidal-probe atomic-force microscopy to investigate the viscoelastic response and interfacial mobility of air-water interfaces in the presence of trace surfactant contaminants. By analyzing frequency-dependent hydrodynamic forces acting on an oscillating microsphere near the interface, we quantify both the viscous drag and elastic restoring forces arising from Marangoni stresses and interfacial deformation. To rationalize these observations, we further develop a disk-bubble analogy, which yields analytical expressions for the interfacial impedance and provides a simple model to connect slip boundary conditions with capillary elasticity. Our experimental observations reveal a pronounced coupling between hydrodynamic stresses and capillary deformation, accurately described by a Kelvin-Voigt rheological model. We demonstrate how minute surfactant contamination significantly reduces hydrodynamic drag and induces a restoring force, immobilizing the interface through Marangoni-driven rigidity, and how the capillarity responds to the hydrodynamic pressure at small gap distances. The findings provide crucial insights into interfacial mechanics, highlighting the delicate interplay between impurity-induced stresses, capillary deformation, and fluid mobility, and paving the way for advanced interfacial characterization methods in microfluidic and biomedical applications.
