2.2. Hydrophilicity
The hydrophilicities of CA nanofibers, PVB nanofibers, CA//PVB Janus nanofibers and photothermal materials are measured, as presented inFigure S3 . CA nanofibers can completely absorb the water droplet within 0.5 s, manifesting their very high hydrophilicity. On the contrary, the water droplet shows a stable contact angle of about 130 ° on hydrophobic PVB nanofibers, which indicates that PVB nanofibers alone are not a suitable host material for constructing solar steam generators because water cannot permeate through PVB nanofibers. As for the CA//PVB Janus nanofibers, the water droplet is slowly absorbed, which brings an important benefit: the water supply rate is not too high in SSG process. Previous studies have found that too fast water supply, normally caused by too high hydrophilicity of host materials, results in decreased temperature of evaporation surface and reduced water evaporation rate.[41] Additionally, the hydrophile/hydrophobe amphipathic CA//PVB Janus nanofibers can float on water surface, whereas CA nanofibers cannot. Therefore, from the perspective of hydrophilicity, CA//PVB Janus nanofibers should be more applicable for constructing solar steam generators compared with CA nanofibers and PVB nanofibers. As seen from Figure S3d, the water droplet is quickly absorbed into photothermal materials, which is because the used CNTs, SiO2 NPs and PDA are hydrophilic. Hence, in the prepared solar steam generator, the water pumped through the CA//PVB Janus nanofibers aerogel can quickly and uniformly spread all over the outer photothermal materials layer owing to the high hydrophilicity of the photothermal materials, which is conducive to water evaporation.