The role of solid surface structure on dropwise phase change processes Manas Ojha a , Arya Chatterjee a , Frank Mont b,c , E.F. Schubert b,c , Peter C. Wayner Jr. a , Joel L. Plawsky a, * a Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, United States b Future Chips Constellation, Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, United States c Future Chips Constellation, Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180, United States article info Article history: Received 1 May 2009 Received in revised form 24 October 2009 Accepted 24 October 2009 Available online 16 December 2009 Keywords: Roughness Contact angle Evaporation Condensation Nanorod abstract We compared the phase change behavior of a partially wetting fluid, nonane, on various SiO 2 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 SiO 2 ; and two surfaces where SiO 2 nanorods had been deposited using glancing angle depo- sition (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 evapo- rative heat transfer. Surface alteration at the scale over which intermolecular forces dominates the fluid-solid interaction provides a convenient means for probing those interactions. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Heat and mass transfer phenomena occurring at the three- phase contact line control processes such as boiling, evaporation, condensation, surface coating, fuel cell performance, and various aspects of transport in biological systems. Phase change has the po- tential to address the cooling requirements of future electronic, photonic, and MEMS devices. As feature sizes in microelectronic and photonic systems get smaller, higher heat loads are generated during their operation. Heat extraction from such devices has be- come a major issue limiting their performance and liquid–vapor phase change systems may be the only way to address the problem. The change-of-phase heat transfer process can be visualized in terms of two resistances in series – (1) a conduction resistance through the liquid film that changes with the thickness of the li- quid film; and (2) a liquid–vapor interfacial heat transfer resis- tance, that depends on the strength of the intermolecular forces between liquid, vapor, and solid, that decreases rapidly with increasing film thickness, and that can be represented as a function of the interfacial temperature jump and an interfacial pressure jump. An evaporating meniscus can be divided into three regions on the basis of film thickness and interfacial curvature. At the ad- sorbed film region where the film thickness is small (<50 nm or so), the intermolecular forces are very strong. This leads to a very high thermal resistance and there is no evaporation in this region. The resistance in the bulk fluid region (>300 nm or so) is also high due to the large conduction resistance through the relatively thick liquid film. The overall thermal resistance reaches a minimum in the transition or contact line region (100 nm) where the small film thickness offers only moderate conduction resistance yet the film thickness is large enough for intermolecular forces to be minimal. A similar argument can be made for condensation where the com- bination of relatively low conduction resistance and high curvature in the transition region leads to a minimum in the vapor pressure and maximum in the condensation rate. An increase in the length of the contact line (due to an increase in the extent of the transition region) should increase the effective heat transfer. This can be accomplished by mechanical dispersion as in spray cooling. Exper- iments by Horacek et al. [1], shown in Fig. 1, demonstrate the di- rect connection. The length of the contact line or the area of the contact region can be increased for individual drops by lowering the contact angle of the liquid. As shown in Fig. 2, the liquid with a lower apparent contact angle, h a , has a larger length of the transition region, L 1 , thus providing a larger area for evaporation or condensation. At the scale of an individual drop, mechanical dispersion is no longer 0017-9310/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2009.11.033 * Corresponding author. E-mail address: plawsky@rpi.edu (J.L. Plawsky). International Journal of Heat and Mass Transfer 53 (2010) 910–922 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt