Dynamic wetting and boiling characteristics on micro-structured and micro/nano hierarchically structured surfaces

In this study, we conducted dynamic droplet wetting tests and saturated water pool boiling experiments on micro-structured surfaces with well-ordered micro-sized pillars (MP) and holes (MH), as well as on hierarchically structured surfaces with nano-sized wires on the pillars (MN-P) and holes (MN-H). The dynamic wetting tests revealed that surface morphology significantly affected both the wetting behaviour and state of the test surfaces. On the MP surface, a Cassie-Baxter state was formed by air trapped between the micro-pillars, and droplet rebounding occurred. However, Wenzel or mixed-state wetting occurred on the MH, MN-P, and MN-H surfaces. In addition, the nano-wires enhanced the surface energy and magnified the liquid imbibition parameter of the hierarchically structured surfaces. The dynamic wetting characteristics of the test surfaces significantly affected the boiling critical heat flux (CHF) values. However, the CHF results could not be explained only by the dynamic wetting behaviours of the test surfaces. Thus, in order to reasonably describe the CHF results, we investigated the experimental results with regard to changes in surface energy, bubble nucleation, and capillary pressure potential resulting from the different surface morphologies.