Characterization of acoustic droplet formation in a microfluidic flow-focusing device.

Local control of droplet formation with acoustic actuation in a microfluidic flow-focusing device is investigated, and the effects of acoustic voltage, frequency, flow-rate ratio, fluid viscosity, and flow vorticity are characterized. Acoustic actuation is provided to affect droplet breakup in the squeezing regime by imposing periodic oscillation to the fluid-fluid interface and, therefore, a periodic change in its curvature at the cross-junction of the device. Time reduction is observed for the three key stages of droplet breakup in the squeezing regime: dispersed phase flow-front advancement into the orifice, pressure buildup upstream and within the orifice together with liquid inflation downstream, and finally the thinning and pinch-off of the liquid thread. It is found that acoustic actuation has less of an effect on droplet size for the continuous phase with a higher viscosity due to the restrained interfacial vibration under a high shear stress environment. Periodic velocity flow fields within the dispersed phase at different phases of one oscillation cycle are calculated based on the results from phase-averaged microresolution-particle-image velocimetry (μPIV). The oscillation paths for the points of maximum vorticities of phase-averaged velocity components are traced, which reveals that the motion is mainly along the y direction.