# Novel Janus Fibrous Membranes with Enhanced Directional Water Vapor Transmission
## 1 Introduction  
Directional water vapor or moisture transport has received extensive attentions and has driven scientists to develope materials with multi-functions and techniques for many kinds of applications, such as fog harvesting, functional fabrics or textiles, and so on [1; 2; 3; 4; 5; 6; 7]. Efficient water vapor transmission has been considered as an ideal way to provide a proper microclimate to satisfy the personal comfort. Learning from the nature, researchers found that the Namib Desert beetles and Texas horned lizard drive the one-way transmission of water droplets in foggy conditions on their typical bumpy backs with alternating hydrophilic and hydrophobic regions; the honeycomb surface of the scales increases the surface roughness and support more condensation foci to survive [8; 9; 10; 11]. By exploring the creatures with excellent directional water transport ability, tremendous methods in attempting to prepare artificial materials with excellent directional water transport or capture capability have been proposed [12; 13; 14; 15; 16; 17].  
Previous studies revealed that fibrous membrane with wettability gradient or contrast in the thickness direction exhibit unique directional water transport phenomena. [PERSON] et al. reported that hydrophobic-to-hydrophilic gradient across the thickness of a fabric can guide directional water-transport from the hydrophobic side to the hydrophilic side, but no water transport takes place in the opposite direction unless an extra force is applied [18; 19]. Such unidirectional water penetration phenomenon was also reported by our group [20] and referred to as \"water diode\" [21; 22] or \"directional wicking\" [1; 23].  
The unique directional water transport can alternatively be achieved by abruptly changing the wettability from hydrophobicity to hydrophilicity along the thickness of the fibrous membrane, such as Janus membrane which is a \"double-side\" membrane with asymmetric properties on either side of the membrane or at membrane surfaces. These properties are primarily derived from different components and/or structures, and are expressed in the form of hydrophilic/hydrophobic or positive/negative surface charges [24; 25; 26; 27; 28; 29].  
Despite the reports about directional water transport materials, the study on enhancing water vapor transmission capability of fabrics or clothing membranes by directional driven property of water Janus membrane is scarce. Herein, hydrophobic poly [4,4\({}^{\prime}\)-methylenebis (phenylisocyanate)-alt-1,4-butanediol/di(propylene glycol)/plycaprolactone) (PU) and hydrophilic cellulose acetate (CA) that exhibit great possibility for application in efficient moisture permeable fabric design and fabrication were chosen as the raw materials. The hydrophobic-hydrophilic PU/CA Janus fibrous membrane was fabricated by the \"layer-by-layer\" electrospinningning technique which shows facile and excellent maneuverability in constructing asymmetric wettability in the thickness direction of nanofibrous membrane. The water vapor transmission capacities from the hydrophobic side to the hydrophilic side and the reverse direction of the Janus membranes were measured. Furthermore, the electrospinningning time of hydrophilic side by which the thickness can be controlled were taken into consideration to uncover the optimal water vapor transmission capacity.