Formation of micropillars on etched calcite and their biomimetic effect on wettability: Implications for built heritage protection

• Etching with 1 M NH 4 Cl (24 h) results in micropillar arrays on (104) calcite surfaces. • Micropillars induce a Cassie-Baxter regime. • Micropillars impart near-superhydrophobicity (∼140°) to hydrophilic calcite. • Micropillars formation is disclosed by a multianalytical and modeling approach. • Near-superhydrophobicity protects calcitic materials against water-related decay. We demonstrate the possibility of biomimetically altering the wettability of calcite (CaCO 3 ) surfaces by the formation of micropillars through anisotropic dissolution with ammonium chloride (NH 4 Cl) solutions. In situ atomic force microscopy (AFM) analysis of the dissolution of (104) calcite faces shows that the ammonium salt solution induces the nucleation of multiple distorted rhombohedral etch pits (with rounded obtuse steps due to interaction with ammonium ions) that deepen and spread at slower rates than in water. Their coalescence results in dense arrays of c -axis oriented pyramidal micropillars (∼15–30 μm height; ∼3–7 μm width at their base) with { 1 ¯ 02 } polar faces stabilized by interaction with ammonium ions. This etching mechanism for micropillar formation is consistent with the computer simulated dissolution process. Micropillar formation on (104) calcite increases the water contact angle from ∼ 60° to ∼ 140°, a value consistent with simulations using the Cassie-Baxter model. These results demonstrate that the treated calcite surfaces achieve near superhydrophobicity. This simple etching strategy has great potential as an effective biomimetic surface treatment to improve the weathering resistance of carbonate stones, such as limestone and marble, and other carbonate-rich materials used in statuary and the built heritage, as well as in modern construction.