Surface with hierarchical structures to enhance heat transfer performance has been investigated for decades. Here we present a unique two-stage vaporization phenomenon observed when the heated surface is covered to confine the thickness of liquid film. This multi-stage vaporization will be critical for optimization applications in thermal management. The hierarchical surface is prepared using: a) isotropic etching to form micro-pillars, and b) wet-chemical etching to produce silicon nanowires (SiNWs) on sidewalls of micro-pillars. Oxygen plasma was applied to modify their surface wettability from superhydrophobic to superhydrophilic. A glass slide was placed right on top of the structures, covering about half of the heated surface, and mechanically clamped by a stainless-steel block. Therefore, the liquid film is about the same height as the micro-pillars. The hierarchical surface was wetted with deionized water before being heated. There are two clear and separate stages that were observed under an high-speed camera: i) thinning while liquid is vaporizing, and ii) mixture of vaporizing and condensation until total dry-out happens. The detailed dynamics of this two-stage vaporization were explored and reported in this study.

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