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

Abstract

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 phaseaveraged 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.

Microfluidic Production of Multiple Emulsions

“…The drop formation in microfluidic devices occurs mainly in dripping and jetting regimes which are reported in all microfluidic geometries [ 14 , 82 , 83 , 84 , 85 ]. The squeezing mode is mostly reported in T-junctions and flow focusing devices [ 86 , 87 , 88 , 89 , 90 , 91 , 92 ], Figure 2 . In addition, a tip streaming mode, resulting in controlled generation of submicron droplets, was reported in flow focusing devices at high viscosity ratios of the dispersed to continuous phase.…”

Droplet pinch-off in acoustically actuated flow-focusing devices

“…In the case under acoustic actuation, Cheung andQiu (2010, 2011) have shown in the preliminary investigations that the droplet size will be affected by the flow rates, acoustic frequency and amplitude. The two parity diagrams for the correlation of flow rates, acoustic frequencies and signal amplitudes are shown in Figures 3 and 4 which show a good agreement between the experimental data and the data from the correlation model with the value of the coefficient of determination R 2 = 0.89 and R 2 = 0.…”

Active droplet generation in microfluidics

“…Cheung et al used this approach to control the droplet generation. 110,111 The vibration is created by a piezoelectric disc attached on top of the dispersed phase channel, Fig. 6(b).…”

“…31 , E, Y and V are the strain coefficient, applied electric field, elastic modulus, and volume of the piezo, respectively.The droplet size is controlled by tuning the duration of the driving pulse and its voltage. Compared to the previously Classification of the approaches for piezoelectricity control (a) T-junction integrated with piezo bimorph actuator 109 (b) Flow focusing device with piezoelectric disc to vibrate the oil-water interface formed during the droplet generation process110,111 (c) Flow focusing device integrated with interdigital transducer (IDT) to generate SAW 112 (d) T-junction integrated with IDT without the SAW influencing oil-water interface. The dotted lines illustrate the changes after activation of the piezoelectricity control 113.…”