The temperature dependent dynamics and periodicity of dropwise condensation on surfaces with wetting heterogeneities

Hypothesis: Biphilic surfaces, namely surfaces comprising hydrophilic areas with a (super)hydrophobic back-ground, are used in nature and engineering for controlled dropwise condensation and liquid transport. These, however, are highly dependent on the surface temperature and subcooling.Experiments: Here, biphilic surfaces were cooled inside a rotatable environmental chamber under controlled humidity. The condensation dynamics on the surface was quantified, depending on the subcooling, and compared to uniform superhydrophobic (USH) surfaces. Rates of condensation and transport were analyzed in terms of droplet number and size, covered area and fluid volume over several length scales. Specifically, from microscale condensation to macroscale droplet roll-off.Findings: Four phases of condensation were identified: a) initial nucleation, b) droplets on single patches, c) droplets covering adjacent patches and d) multi-patch droplets. Only the latter become mobile and roll off the surface. Cooling the surface to temperatures between T = 2-16 degrees C shows that lowering the temperature shortens some of the condensation parameters linearly, while others follow a power law, as expected from the theory of condensation. The temperature dependent condensation dynamics on (super)biphilic surfaces is faster in com-parison to uniform superhydrophobic surfaces. Nevertheless, within time intervals of a few hours, droplets are mostly immobile. This sets guiding lines for using biphilic surfaces in applications such as water collection, heat transfer and separation processes. Generally, biphilic surfaces are suitable for applications in which fluids should be collected, concentrated and immobilized in specific areas.