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

Original entry:  Nan Niu,  APPHY 226,  Spring 2009

''by Luis M. Fidalgo, Chris Abell and Wilhelm T. S. Huck''

==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==

[[Image:Surface_energy.JPG|560px|thumb|center|Figure 1 (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.]]
[[Image:Sequence.JPG|560px|thumb|center|Figure 2 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.)]]
[[Image:Control.JPG|560px|thumb|center|Figure 3 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<sup>-1</sup>.)]]
[[Image:Incorporation.JPG|560px|thumb|center|Figure 4 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, Fe<sup>3+</sup> and SCN<sup>-</sup> react forming a coloured complex. (Channel 200 umwide, 25 um deep. Hydrophilic pattern approximately 100 um long.)]]

As introduced by the authors, microdroplets formed within microfluidic devices present a unique platform for the miniaturization of chemical and biochemical reactions. Traditionally, fusion of drops in microfluidic systems are achieved by applying electric fields in order to polarize the droplet's interfaces or using particular geometries of the microfluidic systems to force the droplets together. In this article, the authors presented a new methods for combining droplets, and it is based on surface energy patterning inside microfluidic channels that allows the fusion of more than two droplets at a single point. Figure 1 shows the schematic of surface energy patterned microfluidic device fabrication. In Figure 2, the authors show the operation of the device. Droplets of distilled water and a dye solution are formed in a continuous fluorous phase. When these droplets flow past the hydrophilic stripe, they are trapped. If more than one droplet is trapped, they are effectively fused and their contents mixed. The coalesced droplets are monodispersed. 

One point that the authors discussed is that when the droplet moves in a fast velocity, it tends to escape the trap as shown in figure 3. Droplet detachment is similar to droplet formation and is governed by the balance between viscous drag force. The authors found that increasing the outer fluid velocity decreases the volume at which the viscous drag overcomes the interfacial tension and causes a droplet to detach. Figure 4 shows a device where droplets of were formed at three separate flow focusing devices. The authors insisted that increasing the number of such flow focusing devices could potentially provide a tool to combine a large number of components at a single fusion step.