Anisotropic wetting of ZnO by Bi2O3 with and without nanometer-thick surficial amorphous films

Anisotropic formation of equilibrium-thickness Bi2O3-enriched surficial amorphous films (SAFs) on ZnO has been documented [Luo J, Chiang Y-M. Acta Mater 2000;48:4501]. This study further explores anisotropic wetting of ZnO single crystals by Bi2O3-rich liquid with and without SAFs. For Bi2O3 on the ZnO {112¯0} surfaces wherein nanometer-thick SAFs are present in equilibrium with partial wetting drops, the measured (advancing) contact angle decreases with increasing temperature, but it stabilizes at ∼6° above ∼860 °C. In contrast, the contact angle is virtually a constant on the {11¯00} surfaces where SAFs are not present. This observation suggests that wetting in the presence of nanoscale SAFs follows a generalized Cahn wetting model. Faceted ridge formation at the triple lines and associated pinning effects are observed. Observation and analysis of unique two-stage isothermal drop receding kinetics show that complete wetting does not occur up to 1050 °C and that the receding contact angle is estimated to be ∼4° via two methods. Quantitative evaluation of a thermodynamic model shows that the observed “residual” contact angle of ∼4–6° and the extended SAF stability can be explained on the basis of a significant attractive London dispersion force.