Dropwise condensation on single-micro-scale roughness hydrophobic surfaces

Abstract

Dropwise condensation (DWC) usually occurs on superhydrophobic surfaces with dual-scale (or multi-scale) roughness in discrete droplets to avoid liquid film formation, enhancing overall heat transfer efficiency. In this study, we demonstrate that the dual-scale roughness hydrophobic surface is not a necessary condition for DWC to occur. A series of single-scale roughness surfaces of regular arrays of square micro-pillars are fabricated to investigate the effect of surface roughness on the wetting behavior of condensed droplets and DWC. A sequence of the wetting behavior of condensed water droplets larger than 200 μm in diameter on single-scale roughness hydrophobic surfaces: Wenzel state → Cassie state → partial Cassie (Cassie-Wenzel mixed) state → Wenzel state (with the irregular shape of three-phase-contact-line) is observed along with increasing surface roughness (pillar height). An in-house design inverted microscopy system is applied for further confirmation of the wetting behavior of condensed droplets. A tiny Wenzel droplet nucleated grow in the microstructure can be dewetted by colliding with a Cassie droplet. This coalescence-induced dewetting transition can effectively make Cassie DWC occur on these single-scale roughness substrates, providing the surface structure design of condensers for Cassie DWC much easier to implement than on dual-scale roughness surfaces.