# Novel Janus Fibrous Membranes with Enhanced Directional Water Vapor Transmission
## 3 Results and Discussion
### Chemical Characterization and Wettability of As-Prepared Membranes
Accordingly, WCA are 132.5\({}^{\circ}\)\(\pm\) 0.12\({}^{\circ}\), 135.4\({}^{\circ}\)\(\pm\) 0.96\({}^{\circ}\), 136.7\({}^{\circ}\)\(\pm\) 0.40\({}^{\circ}\), and 134.9\({}^{\circ}\)\(\pm\) 0.22\({}^{\circ}\) corresponding to PU-6G, PU-8G, PU-10G, and PU-12G, respectively, indicating the PU fibrous layers are hydrophobic (blue line and inserted photos with dotted frames in Figure 4b). For the CA fibrous layers, the WCA are 83.7\({}^{\circ}\)\(\pm\) 0.12\({}^{\circ}\), 88.7\({}^{\circ}\)\(\pm\) 0.64\({}^{\circ}\), 89.1\({}^{\circ}\)\(\pm\) 0.47\({}^{\circ}\), and 89.5\({}^{\circ}\)\(\pm\) 0.06\({}^{\circ}\) corresponding to CA-6G, CA-8G, CA-10G, and CA-12G which show hydrophilicity for all CA fibrous layers. The polymers natural property and porous membranes with high surface roughness are the main reasons for their hydrophobicity and hydrophilicity. It also can be seen that the different fiber diameters show weak influence on the wettability. Since there is only slight water contact angle changes when the type of electrospun needles nozzles is over 8G for both PU and CA electrospun fibrous membrane, the electrospun needle nozzle type of 8G is chosen in this work. Apart from wettability, in order to detect the interaction between water and the hydrophobic, hydrophilic, as well as the Janus fibrous membranes which play important roles in water vapor transmission, the drawing force of the fibrous membranes toward the water droplet was measured by using a micro-force balance system. According to Figure 4d, two testing types, schematically showing as Type 1 and Type 2, were employed. For type 1, a water droplet (5 \(\mu\)L) was brought to contact the membrane and then pulled off according to which the force between the water droplet and the fibrous membranes was probed. Conversely, for Type 2, the water droplet was brought to contact the membrane without pulling off, and the real-time force change was recorded. Meanwhile, according to Type 2, water spreading or adsorbing time by fibrous membrane can be measured. When measured by Type 1 (Figure 4e), the water droplet contacted the surface and then moved downward, the force gradually increased with increasing the \"moving downward\" distance (D). Once the water droplet separated from the membrane, the force reached a maximum value (F) after which it then decreased abruptly. The recorded corresponding force for the fibrous membranes hydrophobic PU-8G, hydrophilic CA-8G and Janus membrane (testing from hydrophobic PU-8G side) was 479.18 \(\upmu\)N (D = 1.187 mm), 336.18 \(\upmu\)N (D = 1.941 mm) and 569.38 \(\upmu\)N (D = 0.046), respectively. Making a comparation between PU-8G and CA-8G, the hydrophobic PU-8G shows higher adhesive force to water at a smaller D value, exhibiting a larger adhesive capacity to water droplet. For the Janus fibrous membrane, the adhesive force reaches to the maximum value at a very smaller D value, indicating that in addition to the hydrophobic force provided by PU-8G layer, water transport from the top PU-8G layer to the bottom CA-8G layer plays an important role in the rapid increase of adhesive force in short time. Figure 4f shows the testing results based on Type 2. Shorter time to reach a peak force suggests faster wicking or spreading of water into the fibrous membrane. It is necessary to point out that for the fibrous surface with water adsorption capacity, once the water droplet makes contact with the surface, it can be adsorbed or spread gradually. The maximum force value appears when the water droplet contacted on the membrane and drops rapidly in a short time, regarded as a shorter time to reach the peak suggesting water spreading or adsorbing. On the contrary, for hydrophobic surface, there is only weak water absorption capacity caused by hydrophobic capillary force, the typical time-force curve has rarely risen or fallen. The recorded time for Janus membrane was 47.16 s, and the time-force curve falls abruptly which indicates the stronger and faster affinity to water.