Lab on a Chip: Surface-induced droplet fusion in microfluidic devices
by Luis M. Fidalgo, Chris Abell and Wilhelm T. S. Huck
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.
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.