Abstract
We show that a microfluidic flow focusing drop maker can be synchronized to a surface acoustic waves (SAW) triggered by an external electric signal. In this way droplet rate and volume can be controlled over a wide range of values in real time. Using SAW, the drop formation rate of a regularly operating water in oil drop maker without SAW can be increased by acoustically enforcing the drop pinch-off and thereby reducing the volume. Drop makers of square cross-sections (w = h = 30 µm, with width w and height h) that produce large drops of length l = 10 w can be triggered to produce drops as short as l ~ 2w, approaching the geometical limit l = w without changing the flow rates. Unlike devices that adjust drop size by changing the flow rates the acoustic dropmaker has very short transients allowing to adjust the size of every single drop. This allows us to produce custom made emulsions with a defined size distribution as demonstrated here not only for a monodisperse emulsion but also for binary emulsions with drops of alternating size. Moreover, we show that the robustness and monodispersity of our devices is enhanced compared to purely flow driven drop makers in the absence of acoustic synchronization.
Highlights
Droplet based microfluidics has been shown to be a powerful tool for biological[1,2] and synthetic systems[3,4]
We show that we can acoustically shorten the droplet dimension of a drop maker approaching the limit of drop formation in the squeezing[14,15] regime l = w without a change in externally driven flow rate
Drop size and rate in absence of surface acoustic waves (SAW) modulation depend on the flow rates and channel geometry, with droplet length l normalized to the channel width at the junction w in the range of 6.5 < l/w < 13.3
Summary
Droplet based microfluidics has been shown to be a powerful tool for biological[1,2] and synthetic systems[3,4]. Precise adjustment of drop size is essential for many droplet based applications – for example when combining drops to accurately control and track the concentration of molecules in the drops. We introduce for the first time the acoustic control of droplet size in real-time in a continuously producing drop-maker. Using acoustic pulses as short as 1.5 ms we are able to synchronize aqueous drop formation in oil with an external signal given by a programmable signal generator for a wide range of droplet rates and volumes. We show that we can acoustically shorten the droplet dimension of a drop maker approaching the limit of drop formation in the squeezing[14,15] regime l = w without a change in externally driven flow rate. The acousto-fluidic device enables us to provide drops with sizes on demand and at high monodispersity and rate
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