Thermo-adaptive interfacial solar evaporation enhanced by dynamic water gating

Freshwater scarcity necessitates efficient, scalable technologies for water collection and purification. Interfacial solar-driven evaporation is promising but faces challenges including heat dissipation, inefficient water management, and salt fouling that impede long-term, high-efficiency operation. Prior designs often rely on rigid structures and passive day–night cycles, limiting autonomous regulation. The research question addressed here is whether a thermo-adaptive evaporator with switchable water transport can autonomously maintain thin-layer water supply for high-efficiency evaporation while initiating self-cleaning to mitigate salt accumulation. The study proposes a bilayer solar-driven water evaporator (SDWE) that couples a photothermal PDA layer with a thermo-responsive sporopollenin (PNIPAM-grafted) gating layer exhibiting LCST behavior. This system aims to harness temperature fluctuations during operation to self-regulate water transport channels, reduce latent heat consumption by maintaining ultrathin water layers, and trigger bulk-water backflow for salt removal, thus enabling continuous, high-performance evaporation and desalination.

Advances in solar evaporators have leveraged high photothermal materials (e.g., metal nanoparticles, carbon, semiconductors, polymers) and architectures (3D porous, 2D lamellae, 1D columns) to enhance efficiency. Strategies such as rational water channel design, water supply control, and multistage evaporation have improved rates. However, scaling to seawater desalination is challenged by salt fouling in water channels. Mitigation approaches include Janus structures, salt-rejecting designs, contactless configurations, and localized crystallization, which, while effective, often depend on rigid structures and passive cycles that hinder sustained autonomous operation. Stimuli-responsive evaporators (magnetic, ammonia-responsive) have been explored but can require strict external fields or lose function post-contamination. Thermo-responsive materials with LCST behavior offer a route to exploit intrinsic temperature fluctuations during evaporation. Previous work with sporopollenin demonstrated LCST-driven near-surface wettability and interfacial water structure changes via grafted micro-particles, suggesting potential for precise, stable, self-adaptive control when integrated into solar evaporators.

Design and fabrication: A bilayer SDWE was fabricated on nickel foam (NiF, ~200 µm pore size; typical sample 5×5 cm area, 10 mm thickness). Step 1: in-situ polymerization of polydopamine (PDA) on pre-cleaned NiF in water/ethanol/ammonia mixed solvent yielded PDA coatings with tunable morphology: smooth (s-PDA), rough (r-PDA), and porous (p-PDA). p-PDA comprised ~520 nm nanospheres assembling into interconnected microchannels, serving as photothermal and thin-water transport layers. Step 2: a thermo-responsive water-gating layer was formed by dip-coating PNIPAM-grafted sporopollenin (PNm-g-SEC) onto the bottom of PDA@NiF, followed by vacuum drying. Sporopollenin (SEC) was prepared from Lycopodium clavatum pollen by alkali extraction and acidolysis; PNIPAM grafting used free radical polymerization initiated by cerium(IV) ammonium nitrate under N2. PN10-g-SEC microparticles exhibited ~26.9 µm size, characteristic C/N/O distributions (EDS), XPS-confirmed grafting, FTIR bands (amide I at ~1650 cm−1, N–H at ~1550 cm−1), and LCST ~33.2 °C (UV–vis turbidimetry). Raman spectra characterized interfacial water structure below/above LCST. Characterization: SEM imaged PDA morphologies and PNm-g-SEC coatings; EDS/XPS/FTIR verified PNIPAM grafting; UV–vis–NIR measured optical absorption (200–2500 nm); Raman probed hydrogen bonding changes; confocal laser microscopy visualized temperature-dependent wettability and thin-water transport along skeleton; micro-CT (500 nm resolution, KI contrast) quantified internal water layer thickness; contact angles assessed wettability transition (PN10-g-SEC: 2° at 20 °C to 153° at 35 °C). Water wicking rates on PDA-coated plates were measured via high-speed imaging. Evaporation testing: Laboratory solar simulator (1 sun, 1000 W m−2) illuminated 5×5×10 mm SDWEs mounted on polystyrene insulation. Mass loss was recorded by a precision balance (1 Hz sampling) after subtracting dark evaporation (pre-measured for 1 h). Temperatures at top (z=10 mm) and bottom (z=0) were monitored with K-type thermocouples; IR thermography imaged thermal localization. Energy efficiency η was computed as η = m·hγ / (Copt·Po) with measured equivalent enthalpy for thin water. Dynamic salt behavior was studied under elevated flux (up to 5 suns) and varying salinities (0.8–20 wt% NaCl). Outdoor prototype testing used a ~10×10 cm SDWE in an insulated basin with a transparent condenser bearing an omniphobic liquid-like coating to promote droplet shedding; water collection was gravity-driven without external energy. Comparative samples: s-SDWE and r-SDWE (alternative PDA textures), p-SE (without SEC layer), and p-ShE (thin-water supply with superhydrophobic SEC) were tested for benchmarking.

• Photothermal and optical performance: p-PDA@NiF achieved broadband solar absorption of 93.6% (200–2500 nm) with low reflectance (~−5.6%), attributable to PDA chemistry and nanosphere assembly, enabling strong light harvesting and thermal localization.
• Evaporation rates and efficiency: Under 1 sun, p-SDWE reached an evaporation rate of 3.58 kg m−2 h−1, outperforming r-SDWE, s-SDWE, and pure water by factors of ~2.2, ~1.3, and ~5.2, respectively. The solar-to-vapor efficiency reached 93.9%, with quantified losses of ~1.9% (convection), ~1.86% (conduction), and reflection ~5.6% (total energy losses ~13%).
• Thermal localization: Surface temperatures rose rapidly (dry surface up to ~63 °C; during evaporation p-SDWE ~39.6 °C), while bottom water remained cooler (~25.9 °C), yielding >13.7 °C gradient. Removing the SEC layer (p-SE) increased heat leakage, raising bottom water to ~27.4 °C and lowering surface temperature (~36.2 °C).
• Thin-water transport and reduced enthalpy: Confocal and micro-CT confirmed a continuous ultrathin water layer confined to PDA microchannels with thickness ~5.7–7.9 µm (avg ~6.8 µm). Raman and DSC indicated disrupted hydrogen-bond networks and an equivalent evaporation enthalpy of ~−1080 J g−1 (lower than bulk water), facilitating efficient phase change.
• Thermo-responsive gating: PN10-g-SEC exhibited LCST ~33.2 °C and switchable wettability (water contact angle from ~2° at 20 °C to ~153° at 35 °C). Below LCST, hydrophilic gating pumped bulk water through large pores; above LCST, hydrophobic gating blocked macrochannels and directed thin water along PDA-coated skeleton.
• Water transport kinetics: Vertical wicking rates on PDA plates: p-PDA ~10.0 mm s−1, r-PDA ~2.10 mm s−1, s-PDA ~0.7 mm s−1; p-PDA surface free energy ~72.3 mJ m−2 (higher water affinity than r-PDA and s-PDA).
• Dynamic operation and salt management: Under prolonged operation with 10 wt% brine, p-SDWE achieved up to 2.45 kg m−2 h−1 water collection with a distinct self-washing phase triggered by salt-induced temperature drop/light off; accumulated salt dissolved and backflow removed brine within ~40 min, restoring uniform surface temperature (~39.6 °C) and high evaporation. For salinities 0.8–10 wt%, rates remained high (e.g., 3.56, 3.34, 3.22 kg m−2 h−1) with no visible crystals over 8 h. For 15–20 wt%, salt accumulation reduced rate after ~6–6.5 h but self-adaptive washing restored performance.
• Outdoor performance: In field tests (average solar flux ~0.72 kW m−2), p-SDWE delivered ~3.14 kg m−2 h−1 evaporation and ~2.48 L m−2 h−1 purification, with stabilized surface temperatures approaching ~50 °C and efficient condensed water shedding via omniphobic condenser.
• Productivity: The optimized system can produce ~18–22 L m−2 day−1 purified water from brine under 1 sun, enabled by continuous thin-layer transport and autonomous salt washing.

The study demonstrates that coupling a porous PDA photothermal/wicking layer with an LCST-based sporopollenin gating layer enables autonomous, thermo-adaptive water management. Above LCST, the gating layer becomes hydrophobic, suppressing bulk flow through macrochannels and sustaining a thin, capillary-fed water film along PDA microchannels. This thin-layer regime lowers effective evaporation enthalpy and minimizes convective heat loss, thereby boosting solar-to-vapor efficiency. When salt accumulation or reduced illumination lowers temperature below LCST, the gating layer switches to hydrophilic, drawing bulk water into the foam, dissolving surface salt, and promoting convective backflow that clears brine and restores thermal uniformity. Confocal microscopy and micro-CT provide direct evidence of channel-selective thin-layer transport, while Raman/DSC corroborate altered interfacial water structure consistent with reduced phase change enthalpy. The hollow sporopollenin structure further localizes heat by providing thermal insulation. Compared with fixed-channel designs and passive day–night cycles, the dynamic water gating maintains high evaporation rates over varied salinities and operating conditions, addressing heat dissipation and fouling simultaneously and enabling stable long-term operation.

A bilayer solar-driven water evaporator featuring a porous PDA photothermal/wicking layer and a thermo-responsive PNIPAM-grafted sporopollenin gating layer was developed to autonomously regulate water transport. The device maintains a continuous ultrathin water supply at elevated temperatures to minimize latent heat and, upon cooling or salt accumulation, initiates bulk water backflow for rapid self-cleaning. The optimized p-SDWE achieved up to 3.58 kg m−2 h−1 evaporation under 1 sun with 93.9% solar-to-vapor efficiency and delivered sustained desalination across a range of salinities, including autonomous recovery after salt accumulation. Outdoor tests confirmed high evaporation and purification rates with effective thermal localization. The approach provides fundamental insights into interfacial water transport and phase transition in confined microchannels and opens opportunities for advancing solar evaporator designs for desalination, contaminated water purification, and heavy metal removal.

While thermo-adaptive gating mitigates fouling, high-salinity feeds (15–20 wt%) still led to salt accumulation after ~6–6.5 h, necessitating a self-washing phase that temporarily lowers throughput. Performance depends on operating temperatures crossing the LCST (~33 °C) to switch modes; conditions far below LCST may favor bulk flow and increase heat losses. Removing the SEC layer degrades thermal localization, highlighting reliance on the bilayer architecture. Long-term durability under real-world fouling/contamination and scalability beyond the tested foam dimensions were not fully detailed.