Researches on the nucleation behaviors of NH₄Cl crystal on coarse aluminum surfaces

Coarse aluminum surfaces were prepared by etching method, the wettability of NH₄Cl-70wt%H₂O solution on the surfaces was investigated, and then the substrates were undercooled and immerged into NH ₄Cl-70wt%H₂O solution to trigger nucleation of NH ₄Cl crystals. Experimental results indicate that the aluminum surface with step-like micro-morphology shows a transformation from strong wettability to non-wettability during its exposure in the atmosphere, in the same time, nucleation density on the strongly wetting surface is significantly higher than that of the non-wetting surface. A hypothesis is proposed to explain the relationship between the wettability and the nucleation density: the capillary effect helps the solution infiltrate into the structures of the surface with strong wettability and drive the air out of the surface structures, so nucleation is effectively promoted by the sharp corners and hollows of the coarse surface; conversely, the air bubbles trapped in the non-wetting surface structures decrease the number of the effective nucleation sites, and leads to a higher nucleation energy barrier, thus the nucleation density is decreased.