In-situ observation of nucleation on a rough chilling surface in NH ₄Cl-H₂O solution

The nucleation behaviors of NH₄Cl crystal on a rough chilling surface immerged in NH₄Cl-H₂O solution were investigated. A phenomenon was found that the nuclei are formed periodically on certain spots of the undercooled surface. The relationship between the surface roughness and the nucleation site selection was investigated on abrasive paper polished aluminum substrate, and it is discovered that the number of nucleation sites decreases significantly with the decreasing roughness of the polished substrate. Further nucleation experiments were carried out on chemical etched aluminum substrate with regular micro-patterns, the result shows that the selection of nucleation sites was more sensitive to the micro-patterns rather than the surface roughness of the interface, and the interfaces with sharp nanometer structures were feasible for the nucleation on the interface.