Interface-engineered rice starch/shellac composites with hierarchical roughness for sustainable oil-water management and food-contact applications.

Herein, an eco-friendly superhydrophobic rice starch (RS)/shellac composite coating was fabricated by spray deposition onto hydroxypropyl methylcellulose (HPMC) films using a food-safe, ethanol-based formulation. Ball-milled RS was incorporated to construct hierarchical surface roughness, while shellac served as a natural, low surface energy binder and interfacial compatibilizer. Comprehensive characterization by scanning electron microscopy (SEM), three-dimensional (3D) surface profilometry, and Fourier transform infrared spectroscopy (FTIR) confirmed uniform coating formation, enhanced interfacial hydrogen bonding, and a finely tuned micro-nano architecture. The optimized coating (HS-30%-2 h) exhibited remarkable superhydrophobicity, achieving a water contact angle (CA) of 153.1° and a sliding angle (SA) of 2.7°, attributable to the synergistic balance between starch concentration and milling duration. Functionally, the coating demonstrated exceptional water repellency and self-cleaning behavior, with minimal residue adhesion (0% for yogurt and 1.3% for honey). Furthermore, it displayed superior selective oil-water separation performance, with an efficiency exceeding 98% and an oil-absorption capacity of approximately 200%, comparable to commercial absorbent paper. These findings elucidate the interfacial design principles governing starch/shellac assemblies and highlight the potential of this biopolymer-based coating for sustainable food packaging and environmental remediation applications.