Preparation of Calcium Alginate-Based Hydrogels with Precisely Designed Centrosymmetric Geometries for Efficient Water Evaporation in Response to Different Solar Incidence Angles

Hydrogels are popular materials for desalination and can significantly reduce the vaporization enthalpy of water; however, there are few reports on hydrogels with a controllable multilevel structural design for water evaporation. Herein, a calcium alginate and traditional Chinese ink-based evaporator (CIE) are proposed and fabricated using directed freezing technology to construct radial channels, followed by freeze-drying and physical cross-linking. Because of the squeezing of ice crystals and the shaping effect of the PDMS template, the prepared evaporator exhibits a sea-urchin-shaped highly geometrical centrosymmetric structure with numerous multilevel pore channels, which promotes the rapid transport of water under different solar incidence angles as the sun rotates as well as overcomes the structural shrinkage of the hydrogel caused by insufficient water supply. Additionally, the radial channels in the spherical hydrogel overcome the traditional limitation of saltwater being continuously concentrated in the same area where the evaporation rate is the highest. As a result, the urchin-structured CIE exhibits a water evaporation rate of 3.52 kg m^–2 h^–1 at 1 sun irradiation, which is 45.5% higher than that of the unpatterned CIE. This multilevel structural design provides a strategy for the fabrication of an all-day water hydrogel-based evaporator without structural shrinkage under solar irradiation.