Konrad Rykaczewski1 Akshay P. Phadnis1

1, Arizona State University, Tempe, Arizona, United States

In many regions of the world collection of dew could provide an alternative or supplementary source of potable water.1 While engineering of surface topography and chemistry to enhance the rate of dew collection has been extensively studied, the effects of mechanical properties of the surface have barely been explored. This topic is highly relevant, since most materials that are commercially viable for dew and fog collectors are polymers, which are substantially softer than typical metallic condenser surfaces. Encouragingly, Sokuler et al.2 have showed that softer substrates promote higher water droplet nucleation density. Wang et al.3 also recently showed that use of an elastic collector that is stretched by wind can increase water collection efficiency. Motivated by these results, here we discuss the effect of substrate’s mechanical properties on all processes relevant to droplet condensation including nucleation, growth, and shedding. Specifically, we experimentally quantify the effect of hydrophobic silicone substrate’s shear modulus on droplet nucleation density and shedding diameter under vertical orientation. By combining analytical model of substrate’s deformation by drop’s surface tension and Laplace pressure with finite element modeling, we theoretically quantify how soft surfaces affect heat transfer across the droplets. Finally, we substitute these results into overall dropwise condensation heat transfer model.4 Our results indicate that despite the increase in droplet nucleation density, decreasing substrate’s shear modulus below 500 kPa decreases the overall condensation rate. This effect is primarily due to additional thermal resistances posed by liquid within substrate depression caused by capillary forces of micro-droplets. Consequently, our work indicates that attention must be paid to mechanical properties of the material when selecting polymeric materials for dew collectors.

1.Tomaszkiewicz et al. Env. Rev. 24, 2015.
2. Sokuler et al., Langmuir 26, 2010.
3. Wang et al. ACS App. Mater. Inter. 9, 2017.
4. Phadnis and Rykaczewski, Langmuir, 2017.