Surface design for dropwise condensation : a theoretical and experimental study

Abstract

The manipulation of the water wetting properties of heat exchangers into dropwise condensation by the use of microstructured surfaces promises an enhanced heat transfer. In order to design a hydrophobic surface geometry, different theoretical models have been introduced in the past. While these models describe the surface-drop-interaction of sessile drops reasonably well, nucleation and droplet growth in dropwise condensation are not considered. Modifications of roughness based models have been introduced as quantitative surface structure design guidelines. The theory of local energy barriers has been contrived and dependencies on the bond number and solid-liquid fraction have been found. This study aims at validating these theoretical models and their applicability for the design of hydrophobic surfaces used for dropwise condensation. To gain deeper understanding of the underlying mechanism of dropwise condensation silicon-nanopillars of five microns height, different diameter (several hundred nanometers) and pillar distance (below two microns) were fabricated in a cryogenic deep reactive-ion etching process. The influence of material properties on the wetting behavior was simulated by using coatings with different intrinsic contact angles (silicon dioxide, Parylene C, octafluorocyclobutane) on the microstructures. The intrinsic advancing contact angle and the surface geometry showed a strong influence on the droplet formation. The investigated theoretical models were not fully coherent with gained experimental data. The experimental results and theoretical simulations show that a simple and conclusive model is yet to be found that describes the droplet-surface interaction during dropwise condensation.