Heterogeneous nucleation and influence of surface structure and wettability

This work explores the physical mechanisms governing heterogeneous nucleation and leads to a new area of bubble nucleation study.; Gas nucleation and pool boiling experimental facilities have been fabricated. The fluids used for this study, water and ethanol, are moderately wetting and highly wetting, respectively. Extensive gas nucleation experiments have been carried through on copper and stainless steel surfaces. In order to compare with the gas nucleation data, nucleation site density has also been measured for saturated pool boiling on brass and stainless steel surfaces with the same surface finishes. Detailed measurements of the surface microstructure have been made using a vertical scanning interferometer. A criterion for identifying potential cavities from surface topography data on surfaces has been provided. Distributions for cavity sizes and half-cone angles have been identified and a statistical model has been used to predict the resulting nucleation site density.; It has been observed that nucleation sites are substantially more populated on the brass surface compared with the stainless steel surface. The variations in the surface structure play an important role in the observed difference. However, due to the large uncertainty in the probability of finding cavities that satisfy the gas trapping criteria, the statistical method does not appear to be useful for predicting nucleation site density on surfaces with randomly distributed cavities.; Using ethanol it has been observed that few sites are active on the brass and the stainless steel surface during gas nucleation experiments. However, nucleation sites readily form in large concentration on both the brass and stainless steel surfaces during pool boiling. The comparison of nucleation site density for pool boiling and gas nucleation casts doubt on the general assumption that heterogeneous nucleation in boiling systems is exclusively seeded by vapor trapping cavities.; Artificial cavities with various sizes and shapes have been manufactured from silicon to study the bubble incipience. The incipience conditions observed for water using bath gas nucleation and pool boiling support Cornwell's contact angle hysteresis theory for vapor trapping cavities. The results suggest that the theoretical considerations required for a deterministic model for incipience from vapor trapping cavities during boiling is more complex than previously hypothesized.