The role of solid surface structure on dropwise phase change processes

We compared the phase change behavior of a partially wetting fluid, nonane, on various SiO2 surfaces that had been modified to alter their roughness at the nanoscale. We compared a total of four surfaces: an as-received, smooth surface; a surface roughened by plasma-enhanced chemical vapor deposition (PECVD) of SiO2; and two surfaces where SiO2 nanorods had been deposited using glancing angle deposition (GLAD). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the surfaces. The topography of the rough surface controlled the wetting characteristics of the fluid that in turn, controlled the change-of-phase heat transfer rate. The measured apparent contact angle characterized the wetting property during the phase change process. Surface roughness promoted wetting in this system, but the direction of heat transfer controlled the topographic design required for enhanced performance. A comparison between two nanorod coatings of differing heights shows that the longer nanorod coating (30 nm high) acted somewhat like a porous surface promoting condensation heat transfer while the shorter nanorod coating (10 nm high) was much more effective at promoting evaporative heat transfer. Surface alteration at the scale over which intermolecular forces dominates the fluid-solid interaction provides a convenient means for probing those interactions.