Lab on a Chip: Surface-induced droplet fusion in microfluidic devices

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Abstract

In this article, the authors demonstrated a new method for droplet fusion based on a surface energy pattern on the walls of a microfluidic device. According to the authors, this new method does not require active elements nor accurate synchronization of the droplets and it is compatible with standard device fabrication techniques, which provides a convenient mean for future applications. Through doing experiment, a new approach for microdroplet control in microfluidic devices is obtained. All in all, the authors stated that surface modification can be used to induce fusion of several previously formed droplets. Last but not least, the authors insisted that this method allows fusion of more than two droplets at a single step and potentially the incorporation of any desired number of components at once.

Experiment

(a) Schematic of surface energy patterned microfluidic device fabrication. Glass supported PDMS substrates previously infiltrated with benzophenone are covered with a solution containing acrylic acid, exposed to UV light and sealed to PDMS moulded microchannels. (b) Micrograph of a microfluidic channel containing a patterned poly(acrylic acid) structure that was stained with toluidine blue. (c) Schematic of a PDMS microfluidic device containing a hydrophilic pattern. Droplets of different components are formed at a double T-junction, and when they encounter the hydrophilic pattern they are trapped, fused and effectively mixed.
Sequence of surface induced droplet fusion. Droplets of different components approach the hydrophilic pattern (located at the center of the dashed rectangle) (t = 0 ms) before they are trapped and fused (t = 0.9, 1.6 ms). A new droplet combination of both is released (t = 4.6, 6.1 ms). (Channel is 50um wide and 25um deep. Hydrophilic pattern is approximately 100um long.)
Control of surface induced droplet fusion by variation of fluid velocity. The upper series of micrographs shows the hydrophilic pattern retaining different amounts of water for different oil flow rates. The lower series of micrographs shows the resultant water droplet released for each flow rate. (Channel 200 um wide, 25 um deep. Hydrophilic pattern approximately 100 um long.Water flow rate 5ul h-1.)
Incorporation of several components into a droplet via a single fusion event. Three different streams of droplets are generated at independent flow focusing devices and combined at the hydrophilic pattern forming droplets containing the three components. After fusion, Fe3+ and SCN- react forming a coloured complex. (Channel 200 umwide, 25 um deep. Hydrophilic pattern approximately 100 um long.)