We introduce an approach and describe the process of acoustically driven formation of droplets in a microfluidic T-junction. Our system allows for fast and precise control of drop volume over a wide range that is fully electrically triggered. We exploit the interaction of a surface acoustic wave (SAW) excited on a piezoelectric, transparent substrate with the fluid to adjust the size of drops in a continuous microflow in real time and relate SAW intensity and drop size. Our device operates in the squeezing regime at low capillary numbers. We describe the mechanism of SAW modulated formation of a monodisperse microemulsion that forms the basis for the integration of more complex operations useful for droplet fluidic systems.
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The wave's mean momentum flux is equal to its mean energy density , which is calculated from sound velocity c0 = 673.6 m/s, wave power PSAW, and sound beam area A = 0.02 mm2 (area estimated from SAW width ≈130 μm and attenuation length 150 μm): . Assuming that the whole momentum is transferred to the fluid at a Rayleigh angle of θ = 10°, the downstream pressure increases by .
