Abstract: The heterogeneous nucleation behaviors of NH₄Cl crystal on a rough chilling surface of aluminum immerged in NH₄Cl-H₂O solution were experimentally analyzed and the relationship between the surface roughness and the nucleation site selection behaviors on polished aluminum substrate was investigated, and it was discovered that the number of nucleation sites decreases significantly with decreasing the roughness of the polished substrate. Further nucleation experiments were carried out on chemically etched aluminum substrate with regular micromorphology on its surface. It has been shown that both the micro-morphology and the wettability vary with the substrate surface prepared by different etching process. The prepared surface with step-like structures has a strong wettability with NH₄Cl-70 wt% H₂O solution and the nucleation density of NH₄Cl on the its surfaces is significantly higher than that of the reference surfaces, which shows that the geometrical morphology features have important effects on both the wettability and the nucleation behaviors.

Key words: Nucleation behavior, Surface structure, NH₄Cl-H₂O, Wettability