Achieving dropwise condensation is of continued interest and desirable in many industrial applications, such as water-harvesting, desalination, power generation, and thermal management. Various non-wettable engineering surfaces have been created to enable formation of discrete condensate droplets and to enhance water shedding from the surface once condensed. Noticeably, previous studies have shown that lubricant-infused surfaces (LIS or SPLIPS) can promote stable dropwise condensation and improve heat transfer rates due to a low nucleation energy barrier and high droplet mobility (low contact angle hysteresis of ≈ 2°), leading to high sweeping rates. However, the nucleation dynamics on lubricant-infused surfaces are considerably less studied and the interplay role of the lubricant film and nucleation remains elusive. Since microscopic condensate droplets (\< 10 μm) account for nearly 80% of the total heat transfer during dropwise condensation, it becomes critical to achieve continuous high nucleation rates to improve heat transfer. In this work, we fabricated LIS by first spraying the commercially available Glaco coating on a glass slide to render it superhydrophobic, and then infusing it with Krytox oils of varying viscosity (38 cSt to 882 cSt, 20 °C). Then, water vapor condensation experiments were conducted in a custom-designed environmental chamber with controllable vapor and substrate temperatures at atmosphere pressure. Based upon our previous finding of lubricant film redistribution during condensation, we use high-speed imaging and optical microscopy to determine the effect of lubricant distribution on the re-nucleation density. We show that nucleation predominantly occurs in the lubricant-poor regions. The relative area ratio of lubricant-poor regions to total surface area, *i.e.* the area of preferred nucleation, is characterized for the different lubricant viscosities during condensation. We find that lowering the lubricant viscosity from 882 to 38 cSt can increase the average area ratio of oil-poor regions from 9% to 20%. Furthermore, we statistically analyzed the new emerging droplets for different the lubricant viscosities at different vapor supersaturation ratios (SR). The results show that the nucleation rate density (NRD) dramatically increases with vapor supersaturation. Surprisingly, we find that lubricant viscosity influences the NRD strongly at high supersaturation (SR \> 2.5), whereas the viscosity has little effect on the NRD at lower supersaturation. At high supersaturation, condensed droplets of all sizes display higher mobility on the low viscosity lubricant, leading to a higher renewal and re-nucleation frequency of surface. Using infrared camera, we experimentally reveal that the temperature of surfaces beneath droplets is lower than in droplet-free regions. Hence, relatively cold surfaces continuously become subjective to the saturated water vapor behind moving and coalescing droplets. This lower temperature decreases the energy barrier for the formation of a nucleus cluster and hence significantly increases the nucleation rate. Similarly, temperatures are lower in the lubricant-poor regions, favoring nucleation there as compared to lubricant-rich regions. This work will help inform the selection of a lubricant for LIS to achieve higher nucleation rate densities, enhance condensation heat transfer, and advance our understanding on the mechanism for droplet nucleation on LIS.


Condensation Nucleation Dynamics on Lubricant-Infused Surfaces

Division / Sponsors: 
Heat Transfer Division (HTD), Fluids Engineering Division (FED)

Heat Transfer Summer Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2020/volume/83709

18th International Conference on Nanochannels, Microchannels, and Minichannels
Paper Proceedings: https://asmedigitalcollection.asme.org/ICNMM/ICNMM2020/volume/83693

Fluids Engineering Division Summer Meeting
Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2020/volume/83716

HT-ICNMM2020 - Virtual Conference, SHTC - Virtual Conference, FEDSM2020 - Virtual Conference

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