Dropwise condensation for improved heat transfer in industrial applications

We focus on the design of copper surfaces intended for industrial applications that involve heat exchange through condensation. It has been shown that dropwise condensation exhibits higher heat transfer coefficient compared to filmwise condensation. Additionally, increased droplet mobility is strongly associated with heat transfer improvement.

To this end, employing fundamentals in wettability engineering and materials science we create diverging cavities in the micro-nano scale followed by low surface energy coating to induce condensate droplet self-ejection from the surface under vapor flow. Based on our preliminary study, nanoneedles which mimic the water strider’s leg can play a key role in droplet self-ejection. After the onset of condensation, small droplets grow in size and finally cover the cavity resulting in the formation of two menisci at the lower and upper part of the droplet. Due to the different radii of curvature a pressure difference between the inside and the outside of the droplet is caused (Figure 1) resulting in a force exerting on it (Figure 1D).

Thus, this force induce motion to the droplet, due to capillarity, towards the divergent part of the space formed by the nanoneedles. To materialize the proposed structure, wet chemistry etching (Aqueous solution of Sodium Hydroxide and Ammonium Persulfate) was used to grow nanoneedles (Copper Hydroxide) on a plain copper sample (Figure 2A). Finally, the surface energy of the fabricated sample is reduced with the use of dipping technique in a ethanolic solution of perfluorodecanethiol which forms a hydrophobic self-assembled monolayer (SAM). The fabricated surface was tested with the use of ESEM (Environmental Scanning Electron Microscopy) to qualitatively assess its performance in terms of droplet mobility and showed satisfactory performance (Figure 2B). Moreover, using an in-house built condensation loop, heat transfer measurements were carried out under vapor flow.

Measurements indicated a considerable improvement of heat transfer coefficient h compared to a smooth surface (approximately h[nanoneedles] ~2 x h[smooth]).

Figure 1: (A)-(C) The water strider and its leg structure in the micro-nano scale. (D) Principle of droplet self-ejection in the strider’s leg.

 
Figure 2: (A) Copper nanoneedles with the employment of wet chemistry etching. (B) ESEM experiments of the nanostructured copper-based superhydrophobic surface.