Near Field Condensation
Dropwise condensation represents the upper limit of condensation heat transfer. Promoting dropwise condensation relies on surface chemical functionalization, and is fundamentally limited by the maximum droplet departure size. A century of research has focused on active and passive methods to enable the removal of ever smaller droplets. However, fundamental contact line pinning limitations prevent gravitational and shear-based removal of droplets smaller than 250 µm. Here, we break this
... break this limitation through near field condensation. By de-coupling nucleation, droplet growth, and shedding via droplet transfer between parallel surfaces, we enable the control of droplet population density and removal of droplets as small as 20 µm without the need for chemical modification or surface structuring. We identify droplet bridging to develop a regime map, showing that rational wettability contrast propels spontaneous droplet transfer from condensing surfaces ranging from hydrophilic to hydrophobic. To demonstrate efficacy, we perform condensation experiments on surfaces ranging from hydrophilic to superhydrophobic. The results show that near field condensation with optimal gap spacing can limit the maximum droplet sizes and significantly increase the population density of sub-20 µm droplets. Theoretical analysis and direct numerical simulation confirm the breaking of classical condensation heat transfer paradigms through enhanced heat transfer. Our study not only pushes beyond century-old phase change limitations, it demonstrates a promising method to enhance the efficiency of applications where high, tunable, gravity-independent, and durable condensation heat transfer is required.