Droplet and Fluid Gating by Biomimetic Janus Membranes

Abstract

wileyonlinelibrary.comOur 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. Results and DiscussionIn order to demonstrate the tunability of gating properties in Janus membranes, two approaches were incorporated, i.e., (1) to hydrophobize selectively one side of an initially hydrophilic membrane, (2) to hydrophilize selectively one side of an initially hydrophobic membrane. These approaches led to a different hydrophilic/hydrophobic balance across the membranes, which allowed complementary liquid gating properties. In the fi rst approach, vapor of 1 H ,1 H ,2 H ,2 H -perfl uorooctyltrichlorosilane (POTS), which previously had been used to hydrophobize cellulose [ 11 ] and silica [ 12 ] aerogels, was allowed to topochemically react on one side of hydrophilic cotton fabric membrane ( Figure 1 a and Supporting Information, Figure S1). CottonThe ability to gate (i.e., allow or block) droplet and fl uid transport in a directional manner represents an important form of liquid manipulation and has tremendous application potential in fi elds involving intelligent liquid management. Inspired by passive transport across cell membranes which regulate permeability by transmembrane hydrophilic/hydrophobic interactions, macroscopic hydrophilic/hydrophobic Janus-type membranes are prepared by facile vapor diffusion or plasma treatments for liquid gating. The resultant Janus membrane shows directional wate...

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“…They are prepared by forming either a lyophilic-to-lyophobic gradient throughout the fabric thickness or a lyophilic/ lyophobic Janus surface on the fabric. [ 4,[6][7][8][9][10] The unique applications of the directional fl uid-transport thin porous media on oil-water separation, [ 9,10 ] fog collection, [ 4 ] and fi ne bubble aeration [ 7 ] were uncovered. [5][6][7] Apart from Janus fabrics, Janus porous membranes with directional fl uidtransport features were reported.…”