Use of diffusion controlled drop evaporation equations for dropwise condensation during dew formation and effect of neighboring droplets

Abstract

In this study, we determined that the equations derived for the diffusion controlled drop evaporation processes can be successfully applied to the condensation rate of water droplets on polymer surfaces having a surface temperature just below the dew point. The differences between the growth rates of condensed isolated droplets on five different polyolefin surfaces, whose surface free energies were in a close range of 30–37mJ/m2, were attributed to the difference in surface roughness which alters the droplet nucleation rate, drop pinning effect and initial contact angles. The condensation rate of isolated droplets decreased with the increase of surface roughness, water contact angle and contact angle hysteresis in the order of ethylene–vinyl acetate copolymer (EVA)>low density polyethylene (LDPE)>high density polyethylene (HDPE)>polypropylene–polyethylene copolymer (PPPE)>polypropylene (PP). The drop radius of the individual isolated droplets was found to grow according a power law with exponent 1/3 except PP similar to previous reports. We also compared the volume increase of an isolated single droplet with the volume increase of another similar sized single droplet which was surrounded by neighboring droplets and found that when neighboring droplets are present in close proximity, the condensation rate was 14–40% lower than that of a single isolated droplet due to blocking of lateral water vapor diffusion. This effect was more pronounced on substrates having high surface roughness.