Droplet and Fluid Gating by Biomimetic Janus Membranes

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 hydro-phobic molecules from the hydrophilic outer side across the hydrophobic core layer whereas hydrophilic polar mole-cules 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 rec-tify gating of liquids. Instead of a mem-brane 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. Accord-ingly, 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 col-lect water droplets from oil or oil droplets from water.