Effect of CaCO3 Particle Size on Surface Wetting and Adhesion: Studies on PMMA Model Substrates and Laurus nobilis Leaves

Leaf surfaces are protected by a hydrophobic cuticle with variable chemical composition and roughness, which often limits spray droplet retention and absorption. Optimizing foliar spray performance is therefore critical to maximize the desired effect on the target plant and minimize environmental impact. This study investigates the impact of particle size of calcium carbonate (CaCO₃) in the presence and absence of a non-ionic surfactant on leaf surface deposition and wetting behavior. The tested formulations contained (i) no particles, (ii) CaCO₃ nanoparticles, and (iii) CaCO₃ microparticles (each at 2 wt%), applied using an airbrush or a handheld sprayer to polymethyl methacrylate (PMMA) plates, serving as model substrate, and on laurel leaves (<i>Laurus nobilis</i>). Water contact angle (WCA) measurements and coverage analysis were used to assess wetting performance. Initial WCA values were low (<12°) for all coatings, but rinsing revealed distinct behaviors. Coatings with nanoparticles retained a low WCA (<40°) and high coverage (>60%) after multiple rinsings, whereas microparticle coatings showed a sharp WCA increase (>60°) and significant coverage loss after few rinses. These findings demonstrate the long-lasting wetting effect of CaCO₃ nanoparticles and highlight their potential as additives to enhance spray formulation performance.