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.