Droplet pinch-off in acoustically actuated flow-focusing devices

The evolution of the interfacial profile for one complete droplet formation process in a flow-focusing device was investigated. Two critical curvature values that represent the liquid thread detachment from walls and the onset of pinch-off were discovered. The rate of curvature evolution depends on the flow-rate ratio between the two immiscible fluids and was modulated by acoustic actuation. Three distinct evolution regions which represent, respectively, the periods before and after the liquid thread detachment from walls and after the onset of pinch-off till final breakup are shown. In the case under acoustic actuation, the droplet formation process is accelerated; both the time till the liquid thread detachment from walls and the onset of pinch-off are speeded up. Phase-averaged two-phase micro-particle-image-velocimetry (mu PIV) was applied to extract the oscillating flow fields in the two immiscible phases. Periodic contracting and expanding streaming flow fields in the dispersed phase and steady streaming flow fields in the continuous phase were obtained. The steady streaming flow in the continuous phase provides an additional driving force which helps to drive the continuous phase into the gutters. This additional flow facilitates the onset of breakup and thus smaller droplets are formed upon acoustic actuation.