Condensation on CICNT Structured Superhydrophobic Surfaces

This study experimentally investigates the impact of subcooling magnitude, CNT diameter, time duration of condensation, and amount of non-condensable gases on the nature of dropwise condensation and condensate removal. Hour long experiments took place in a vacuum chamber on a vertically oriented test surface with varying CNT diameter and subcooling magnitude. Thermocouples provided data to compute overall heat transfer magnitudes. Three types of condensation were observed: drop jumping, drop wetting of the CNTs, and surface flooding. Results reveal that drops retain mobility at CNT diameters smaller than ~60 nm and at subcooling temperatures less than ~7C. Here drops are mobile and self-remove from the surface by the mechanism of jumping. As CNT diameter increases to 70-75 nm, the drops lose mobility and become pinned to the surface; the nature of the condensation transitions from droplet jumping to surface flooding. This same transition occurs at subcooling magnitudes greater than 9-10 C, independent of CNT diameter. The amount of time the condensate interacts with the surface also affects drop mobility. After an hour exposure time, the condensation transition occurs at smaller CNT diameters and lower surface subcooling. Results show the influence of time is not due to a degradation of the surface structure. However, when non-condensable gases are introduced to the test chamber, the mechanism of drop jumping is completely suppressed for all conditions explored and only drop wetting or flooding is observed.