Figure 1: Scanning electron micrographs with uniform array of micropillars. The micropillars have the same height of 17 µm and pitch of 25 µm, while the diameters of pillars are (a) 6 µm, (b) 11 µm, and (c) 16 µm, respectively.

The mechanism of critical heat flux (CHF) is commonly attributed to two limits during boiling behavior: 1) the hydrodynamic limit due to Helmholtz instability and 2) the capillary limit determined by surface wettability ^[1] .  In recent years, a significant amount of research has been focused on CHF enhancement by utilizing micro/nanostructured surfaces to improve wettability ^{[2] [3] [4]} , with CHF of ~200 W/cm² being demonstrated ^[4] .  While most works are focused on making small structure sizes to improve surface wettability, the effect of this roughness-augmented wettability on CHF is poorly understood.  The limit of CHF enhancement with roughness-augmented wettability, where hydrodynamic instability becomes the dominant mechanism for CHF, has not been investigated.  In addition, boiling on nanostructured surfaces suffers from the requirement of high superheat due to bubble geometries closer to the homogeneous nucleation limit.  As a result, the heat transfer coefficient (HTC) on nanostructured surfaces is sacrificed ^[3] , which impairs the heat removal capability especially for applications demanding small temperature difference.

In this study, micro/nanopillar arrays are fabricated with a series of pitch and diameter size, as shown in Figure 1.  The sizes of pillar are designed to ensure that bubbles in the Cassie state, where vapor bubbles are suspended on the pillars, are energetically favorable such that bubble detachment is enhanced.  The series of sizes of the structured arrays generate various capillary forces, which allow the study on the mechanism for CHF and the limit of CHF enhancement with roughness-augmented wettability.  Furthermore, the investigation on surface roughness, where hydrodynamic instability dominants, gives the optimal size of structures for CHF enhancement and explores the feasibility of heterogeneous bubble nucleation on surfaces with proper structure geometry to reduce superheat.

1. S. G. Liter and M. Kaviany, “Pool-boiling CHF enhancement by modulated porous-layer coating: theory and experiment,” Int. J. Heat Mass Transfer, vol. 44, pp. 4287–4311, 2001.
2. C. Li and G. P. Peterson, “Parametric study of pool boiling on horizontal highly conductive microporous coated surfaces,” J. Heat Transfer, vol. 129, pp. 1465-1475, 2007.
3. S. Kim, H. D. Kim, H. Kim, H. S. Ahn, H. Jo, J. Kim, and M. H. Kim, “Effects of nano-fluid and surfaces with nano structure on the increase of CHF,” Exp. Therm Fluid Sci., vol. 34, pp. 487–495, 2010.
4. R. Chen, M.-C. Lu, V. Srinivasan, Z. Wang, H. H. Cho, and A. Majumdar, “Nanowires for enhanced boiling heat transfer,” Nano Lett., vol. 9, no. 2, pp. 548-553, 2009.