An interferometric technique to study capillary waves

We describe a new interferometric technique to study gas-liquid and liquid-liquid interfaces. Bubbles and drops are subjected to an alternating electric field which excites capillary oscillations at the interface, if charged. Bubble or drop deformation is detected by the change of the internal optical path of a laser beam crossing perpendicular to the oscillation axis. Due to the closed geometry, a discrete spectrum of stationary oscillation frequencies (normal modes) is excited. The interferometric nature of the measurement and the resonant nature of the oscillation modes concur in allowing for high sensitivity, in the sub-nanometric region. We present a detailed description of the experimental setup and examples of applications of the technique to the study of both gas-liquid and liquid-liquid interfaces, either naked or with adsorbed surfactant monolayers, for bubbles and drops with diameter 1 mm. In particular, the resonance frequencies and the width of the resonance peaks depend on the surface tension and the viscous dampening, respectively. We show that, by this new technique, properties of the interface can be accessed with confidence at the sub-nanometer scale, and surface phenomena, like the monolayer phase transition or the peculiarities of adsorption/desorption processes, can be unraveled in concentration regimes which are too low for existing methods.