Non-contact mechanics of soft and liquid interfaces by hydrodynamic confinement using a frequency-modulated AFM

Measuring the mechanical properties of liquid interfaces without direct contact remains a major experimental challenge, particularly for liquid liquid systems. Here we propose a frequency modulated atomic force microscopy method that probes interfaces through hydrodynamic confinement of a viscous liquid film between an oscillating probe and the interface. The method is first quantitatively validated on a model liquid solid interface, where the measured imepdance and confinement thickness agree with theory over a decade of elastic moduli. It is the aplied to a liquid liquid interface which exhibits a purely viscous response. As a result of the absence of elastic restoring force, the confinement thickness increases to micrometric values. These original measurements demonstrate that hydrodynaic confinement provides a quantitative non-contact probe of liquid interfaces and opens new perspectives for invetigating complex and highly deformable systems.