Passive solar-thermal interfacial crystallizer using laser-textured superwicking black metal

Mechanism

An aluminium panel textured with femtosecond-laser pulses creates an array of micro-grooves carrying nanostructures. This "superwicking black metal" (SWBM) surface absorbs nearly all incident sunlight (~98% at peak, ~92% averaged across the solar spectrum), pulls a thin film of seawater uphill by capillary action, and drives crystallising salt outward off the active surface. As water evaporates, the highest concentration reaches the edge first (coffee-ring effect), salt nucleates there, and salt creeping propagates the crystal outward into an untreated passive region. The active area stays clean while salt accumulates at the edges for simple removal.

Why it fits this issue

• No consumables, no electricity: sunlight only — no RO membranes, no chemical pretreatment, no high-pressure pump, no battery bank. Removes most supply-chain and skill dependencies that kill decentralised systems.
• Self-cleaning on real seawater: earlier solar-thermal evaporators clogged on genuine seawater (Mg/Ca hard crusts); most prior work quietly used simulated brine. This design was demonstrated on Atlantic, Pacific, and Indian Ocean samples.
• Zero liquid discharge: salt harvested as solid; reservoir salinity stayed flat over a 7-day run — nothing dumped back as brine.

Measured performance

• Evaporation rate: 1.76 ± 0.04 kg/m²/h daytime under one sun; ~0.70 kg/m²/h at night.
• ~74% solar-to-vapour conversion efficiency; ~100% of dissolved salt captured as solid.
• Outdoor rooftop test (Rochester, March 2024, 9 cm² panel, 9 h): equivalent to ~10.3 litres/m²/day freshwater plus ~0.38 kg/m²/day solid salt; product water below WHO/EPA salinity limits.
• 7 consecutive days without maintenance; optical and wicking properties intact afterward.

Co-product note (speculative)

A companion study functionalises the same surface with metatitanic-acid nanoparticles to selectively trap lithium, recovering ~50% of lithium from Great Salt Lake salts. Treat as early proof of concept only: seawater lithium is ~0.17 ppm, no commercial-scale direct-lithium-extraction plant is proven, and lithium prices fell ~80% from 2023 peaks. If pursued, it warrants its own issue rather than being folded into the water economics here.

Honest limits

• Maturity: centimetre-scale device, days-to-a-week of testing. Not a product.
• Throughput ceiling drives a scaling area question (see sub-issue).
• Durability and scale-up unproven (see sub-issue).
• Salt logistics: scraping works at 9 cm²; harvesting, storing, and disposing of tonnes of mixed solid salt at deployment scale is an unsolved operational problem the authors themselves flag.