Droplet and Fluid Gating by Biomimetic Janus Membranes

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 6023 wileyonlinelibrary.com Our work is inspired by passive transport across cell membranes, which offer an intriguing example of regulating membrane permeability based on transmembrane hydrophilic/hydrophobic interactions. [ 10 ] The cell membrane has a lipid bilayer structure consisting of hydrophilic phosphate outer layers and hydrophobic hydrocarbon core layer. This structure allows spontaneous diffusion of hydrophobic molecules from the hydrophilic outer side across the hydrophobic core layer whereas hydrophilic polar molecules show reduced permeation. The cell membrane thus passively controls the permeation of molecular-sized entities, and it inspires us to apply hydrophilic/ hydrophobic membrane designs to rectify gating of liquids. Instead of a membrane of nanoscale thickness, we selected technically relevant porous membranes of submillimeter-thickness to construct hydrophobic/hydrophilic asymmetry across it. Taken water transport as an example, such membrane might preferentially allow water to penetrate from the hydrophobic side, but tend to hinder its penetration from the hydrophilic side. Accordingly, we prepare two types of hydrophilic/hydrophobic Janus membranes by facile vapor diffusion or plasma treatments and demonstrate directional gating of water droplets as well as continuous water fl ow in air-water system. More generally, our membranes show directional gating of droplets in oil-water systems with integrated selectivity for either oil or water. Such membranes possessing both selectivity and directionality in liquid gating represent a new concept of intelligent materials. We also demonstrate the construction of “Janus trapper” to collect water droplets from oil or oil droplets from water.