| Incipience wall superheats over super-smooth Si, Al and Ti surfaces were collected at 1atm saturation conditions with FC-72 as the working fluid. Before experimentation, the fluid was put through a 2 hour degassing process to remove vapor. In addition a novel micro-heater was designed and fabricated that provided a constant temperature surface condition with slow heating rates. The average incipience wall superheats were 23.5K for Si, 26.4K for Al and 27.7K for Ti with a variance of less than 0.8K for all three cases. Contact angles were also measured; however, due to the highly wettability of the fluid, the resulting measurements showed large variability. For all cases, incipience occurred at approximately 30% of the homogeneous superheat limit predicted from classical theory. The observed differences in the wall superheat for the three surfaces was small.;The primary surface heating element has a surface area of 250 mu m x 250mum which resulted in a very small Grashof number for the system. Consequently, the temperature profile over the heating element was evaluated from the steady Laplace equation using the Fourier integral method. Based on the critical radius at the time of incipience, it was determined that the temperature gradient over the vapor embryo is very small, and thus it can be assumed that the embryo is approximately equal to the wall temperature.;Finally, since the measured surface roughness is an order of magnitude less than the critical radius for incipience it is concluded that nucleation could not have been initiated due to any vapor trapped within asperities on the surface. A theoretical framework based on homogeneous-like nucleation and the macroscopic contact angle is proposed. By analyzing the free energy barrier in terms of the vapor cluster, the liquid/vapor interface and the solid/vapor interface, the chemical potential at incipience for FC-72 and water can be determined from experimental measurements of wall superheat and liquid/vapor contact angle. Based on measured values of the chemical potential at incipience, the wall superheat at incipience for heterogeneous boiling on smooth surfaces can be determined. |
