Bioinspired Solar Distillation and Salt Precipitation with Capillary-driven Water Transport and Surface Cleaning

Abstract

Solar distillation has great potential for environment-friendly desalination of seawater, while conventional desalination practices employ fossil fuels with significant carbon footprint and discharge brine, which has serious environmental concerns. Passive solar desalination relies on capillary transport of saline water from sea level to the evaporating surface, capturing the main attributes of the transport mechanism in halophytes (e.g., mangroves). In this dissertation, a new constitutive equation is proposed to relate capillary pressure and saturation of wetting liquid for passive transport in porous structures/media. A capillary transport model for liquid imbibition in porous structures with varying liquid saturation is developed by coupling the proposed constitutive correlation with the momentum conservation equation. To verify the validity of the proposed model, this work presents the systematic characterization of the capillary dynamics and height-dependent liquid saturation of water within micro/nanostructured heterogeneous titanium mesh as a synthetic stem.

The capillary transport model is integrated with an evaporation-precipitation model to study the coupled phenomenon of evaporative wicking. The model can quantify the reduction in evaporation flux due to an increase in salinity and light reflection from precipitated salts. The underlying mechanisms of edge-preferred salt crystallization in synthetic and real Mangrove leaves are elucidated by predicting the salt concentration profile along the leaf surface. Moreover, thermodynamic analysis based on classical nucleation theory is performed to study the salt crystallization behavior at the leaf. The analysis shows that the chemical potential difference of salt in solution plays a dominant role in shaping crystallization behavior, as compared to the reduction in nucleation energy barrier owing to the cavities and micro/nanostructure on the synthetic titanium leaf. Moreover, the current study also investigates the effect of leaf spikes on evaporation-driven salt crystallization and its removal. The magnitude of the back diffusion coefficient (which plays a vital role in salt peeling) for titanium mesh is found to be at least one order of magnitude higher than the standard diffusion coefficient owing to the presence of natural convection and the Marangoni effect.

The in-situ characterization of the salt efflorescence (similar to that on titanium-based synthetic leaf) and subflorescence (salt precipitation within the volumetric porous structure) is systematically conducted with Nuclear Magnetic Resonance and Imaging (NMR-MRI). The fully saturated porous media are illuminated under simulated sunlight for 12 hours followed by 12 hours of dark period. The pore blockage due to salt crystallization in pore throats of tight porous media is investigated by monitoring the shift in T2 distributions, particularly minimum T2 as it represents throat size. For coarse porous media, patchy efflorescence takes place at the evaporating surface with distinct pore size distributions, for which the transverse relaxation time of the confined water (within salt patches) approaches the bulk relaxation time.

This dissertation also provides insights into the dynamic interaction of an individual moveable micro-particle with liquid meniscus to understand capillary-driven self-cleaning of dust and secreted salt crystals from solar evaporator. A mathematical model for the particle velocity is developed to elucidate the intertwined physics of the particle and the droplet. The capillary rise of water around a single micro-particle is studied to determine the maximum capillary forces during physical interaction between the meniscus and particle. These investigations on droplet-particle interaction help in understanding the underlying mechanisms of capillary-driven contaminant removal by the condensate or raindrops for self-cleaning solar evaporators and crystallizers.

The revealed physics on capillary wicking, interfacial evaporation-driven salt precipitation, NMR/MRI characterization for pore blockage and meniscus-particle interactions is valuable for the rational design of sustainable solar thermal distillation processes.

Date of Award	10 May 2024
Original language	American English
Supervisor	TJ Zhang (Supervisor)