Interferometric detection of hydrodynamic bubble–bubble interactions

We report a new interferometric method to study the interactions between two gas bubbles undergoing small-amplitude oscillations in a liquid, based on the extension of a previously developed one-bubble set-up. Nanometric oscillations of millimetre-sized supported bubbles are excited acoustically; the response of each bubble is recorded interferometrically, as a function of the mutual distance (from quasi-contact to greater than the bubbles radii). The interferometric nature of the technique and the resonant nature of the vibration modes enable the accurate measurement of the amplitude (with sub-nanometric sensitivity), frequency and mutual phase of oscillation, whose variations over the bubble–bubble distance range allow the interactions to be probed. The bubbles oscillate at the same frequencies, exhibiting a low-frequency, in-phase and a high-frequency, out-of-phase resonance peak, whose separation is a function of distance, in good agreement with the theory for free interacting bubbles. The technique, here demonstrated for the volume modes of air bubbles in water, can be extended to other gas–liquid and liquid–liquid interfaces, bare or adsorbate-covered, as well as to shape oscillations.