Selective transportation of microdroplets assisted by a superhydrophobic surface with pH-responsive adhesion

We report a new strategy to realize the selective transportation of microdroplets assisted by a superhydrophobic surface with pH-responsive adhesion. On the surface, only basic microdroplets can be pinned and acidic or neutral microdroplets can easily roll off. Therefore, by using the surface as a "mechanical hand", microdroplets can be transported selectively according to one's requirements by simply controlling the pH of the solution. The special ability of the surface to achieve selective transportation is ascribed to the following two reasons: 1) superhydrophobicity, which can avoid the wetting problem, and 2) pH-responsive adhesion, which results from the combined effect of chemical variation of the carboxylic acid group and microstructures on the surface. Furthermore, we also demonstrated a process of selective transportation of microdroplets for applications in droplet-based microreactors through our surface. The results reported herein advance a new method to realize the selective transportation of microdroplets and we believe that this method could potentially be used in a wide range of applications, such as biomolecular detection and transportation in biochips.