Janus Droplets and Droplets with Multiple Heterogeneous Surface Strips Generated with Nanoparticles under Applied Electric Field

A novel method of fabricating Janus droplets and droplets with heterogeneous strips is presented in this article. These droplets were made by covering the surfaces of oil droplets with different nanoparticles under an electric field in a microfluidic chip. The microfluidic chip was used to control the delivery of nanoparticles, and the electric field was employed to assemble the nanoparticles adhering on the droplet. By controlling the delivery of nanoparticles in the microfluidic chip under the electric field, different nanoparticles accumulate on the droplet surface and the desired Janus droplets and droplets with heterogeneous strips can be formed. Because of the presence of charged nanoparticles on droplets' surfaces, the Janus droplets and droplets with heterogeneous strips have unique electrokinetic properties. The electro-osmotic flow fields around Janus droplets and droplets with heterogeneous strips in different pH solutions were visualized, and the electrokinetic velocities of these droplets in a microchannel were measured as a function of the electrical field. On the basis of their specific electrokinetic properties, two applications of the droplets coated with nanoparticles were developed: flow focusing and microvalve. The experimental demonstrations indicate that the Janus droplets and droplets with heterogeneous strips offer great potential in sensing, actuating, and controlling fluid flow.