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The present paper reports a novel manipulation method for droplets using acoustic radiation pressure and acoustic streaming. In an acoustic field, droplets deform, oscillate and move in a wide range of applied frequencies. The behavior of a droplet depends on the droplet size, acoustic field and interfacial tension between the two phases. The acoustic field is controlled by the voltage and frequency of the piezoelectric actuator. The results demonstrate a method for low-frequency acoustic actuation of droplets in a microfluidic environment.

This study reports on the breaking up of droplets which can be manipulated with acoustic fields. The oscillation of vortex in a breaking droplet is observed. The droplet size is dependent on the flow-rate combination of the two fluids as well as the frequency and power of the acoustic actuation. Acoustic microstreaming flow is observed in the dispersed phase at the cross-junction of the device. The microstreaming flow causes a stratified vortex flow structure within the dispersed phase. Two stratified vortex centers at the side poles of the droplet are found.Single and double emulsion production technologies have wide applications in bioengineering and food processing; for example, the use of a high-throughput microfluidic chip for polymerase chain reaction in picoliter droplets for the detection and quantification of rare species in a population 1 and the use of silica spheres with a large pore size for fast adsorption of proteins. 2 Microfluidic devices, flow-focusing, and T-junctions with different geometries are applied to the production of single emulsions with different sizes. 3-10 Experimental investigations show that the size of droplets produced by both methods mainly depend on the flow-rate ratios between the inner and the outer fluids with a specific pair of fluids and device geometries under investigation. [3][4][5][6][7][8][9] Other parameters which would also affect the size of the produced droplet include the junction angle of the device, surface tension, viscosity of the fluids, etc. 9 In order to increase the range of droplet size which is only controlled by the flow-rate ratios between the two fluids in most devices, it is suggested that actuators or other means should be integrated with the device so as to achieve local control of the droplet breakup. 11 Attempts to provide local control have been made by Kim et al. 12 and Tan et al. 13 But local control by an electric field as suggested by Kim et al. is only applicable for small flow-rate ratios ͑flow-rate of the inner fluid to outer fluid͒ with a low flow-rate of the dispersed phase. In the present study, we demonstrate the integration of a piezoelectric actuator with a microfluidic flowfocusing device so as to achieve local control of droplet production. Acoustic streaming flow is observed within the dispersed phase and phase-averaged microparticle-imagevelocimetry ͑PIV͒ ͑Refs. 14 and 15͒ is applied for quantitative verification of the flow field at different phases within the dispersed phase.Soft-lithography is adopted for the fabrication of the flow-focusing device ͑Fig. 1͒. 16 SU-8 ͑SU-8 2035, MicroChem Co.͒ is spin-coated with a thickness of about 100 m and is exposed to UV light and developed to give a rectangular microchannel pattern. The stamp resin used to replicate the microchannel is the 10:1 mixture of polydimethylsiloxane ͑PDMS, SYLGARD ® 184, Dow Corning Co.͒ and its curing agent which is degassed in a vacuum chamber before use. The hardened PDMS with punched holes is then bonded to a glass wafer by oxygen plasma bonding.De-ion...