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Linda Zou1

1, Khalifa University of Science and Technology, Abu Dhabi, , United Arab Emirates

Cloud seeding materials as a promising water augmentation technology have drawn more attention recently. We designed and synthesized a novel type of core/shell NaCl/TiO2 (CSNT) particles with controlled particle size, which successfully adsorbed more water vapor at low relative humidity, (from 20 % RH) than that of pure NaCl, deliquesced at lower environmental RH of 62 - 66 % than the hygroscopic point (hg.p., 75 % RH) of NaCl, and formed larger water droplets ~ 6 - 10 times of its original measured size area, whereas the pure NaCl still remained as crystal at the same condition. These benefits of CSNT particles were observed visually through in-situ observation under Environmental - Scanning Electron Microscope (E-SEM). The enhanced performance was attributed to the synergistic effect of the hydrophilic TiO2 shell and hygroscopic NaCl core microstructure. Specifically, as for pure NaCl crystal, the condensation of water vapor molecules occurred at the interface of NaCl crystal and the water vapor, due to the weak hydrophilic property of the NaCl surface, their deliquescence was totally dependent on the environment RH conditions, ie, if the environmental RH value was below 75%, no interaction will happen, only when the environmental RH value was above 75%, the NaCl crystal can deliquesce. Whereas for the CSNT particles, this process was enhanced by the core/shell structure, the hydrophilic TiO2 shell accumulated water molecules around the core, and increased significantly the local RH value near the NaCl to saturation or supersaturation conditions, this promoted the deliquescence process to occur even when the environmental RH value was lower than 75%. Moreover, the critical particle size of CSNT particles (0.4 - 10 μm) as cloud-seeding materials was predicted via Kelvin equation based on their surface hydrophilicity. Finally, CSNT particles were added in the cloud chamber to evaluate their water droplet formation in a three-dimensional environment. It was found that both the water-droplet concentration and the droplet size increased greatly across all size ranges. Especially, at 100 % RH, the concentration of water droplet size between 10 - 25 μm (which is very crucial to the rainfall) caused by CSNT particles was up to 290% more than that by NaCl. These excellent results were highly consistent and positively confirmed that CSNT particles can be a type of effective cloud-seeding materials. This strategy was the first effort to design nano/micro-structured materials for rain enhancement and broad water augmentation technology.

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