Lotus-Effect Hydrophobic Coatings on Engineered Wood: Toward Self-Cleaning Building Envelopes in Humid Climates

Introduction
Maintenance of timber façades in humid climates is challenged by biofouling, staining, and moisture-induced degradation. Inspired by the superhydrophobicity of lotus leaves, this study investigates silica-based nano-rough coatings applied to cross-laminated timber (CLT) panels. By replicating hierarchical surface textures, the coating aims to induce self-cleaning behaviour, reduce water uptake, and inhibit fungal colonization without altering timber’s visual character.

Methods
A sol–gel process was used to deposit bimodal silica nanoparticles (50–200 nm) onto kiln-dried CLT samples via dip-coating. Surface morphology was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Static and dynamic water contact angles were measured per ASTM D7334 to assess hydrophobic performance and contact-angle hysteresis. Fungal resistance was tested by exposing coated and uncoated specimens to Trametes versicolor in a humidity chamber at 28 °C and 90 % RH for 12 weeks. Optical transmittance and colourimetric shifts were recorded with a spectrophotometer to verify aesthetic neutrality.

Results
Coated surfaces exhibited hierarchical micro- and nano-roughness, achieving an average static contact angle of 159 ° and contact-angle hysteresis below 4 °, compared with 65 ° and 22 ° on uncoated CLT. Simulated rain tests showed >95 % removal of surface particulates on coated panels versus <40 % on controls. After fungal exposure, mass loss in coated samples was 1.2 % (±0.3 %), significantly lower than 7.8 % (±1.1 %) in uncoated specimens (p < 0.01). Spectrophotometry indicated ΔE*< 1.5, confirming minimal impact on timber appearance.

Conclusions
The lotus-effect coating delivers superhydrophobicity, effective self-cleaning, and substantial fungal resistance while preserving the natural aesthetics of CLT. Its low application complexity and durability under accelerated weathering suggest strong potential for reducing maintenance loads on timber building envelopes in tropical and subtropical regions. Future work will explore field trials and long-term UV-stability to validate lifecycle performance.