Exploiting Microscale Roughness on Hierarchical Superhydrophobic Copper Surfaces for Enhanced Dropwise Condensation

These studies are limited to silicon substrates, however, and are not suitable for scaled-up industrial applications. Therefore, it is highly desirable to develop hybrid surfaces that are compatible with materials commonly employed for heat transfer applications, such as copper. To the best of our knowledge, studies of dropwise condensation on copper substrates have exclusively focused on nanostructured surfaces; [ 25–27 ] condensation dynamics on copper surfaces with hierarchical surface structures have not been explored. In this work, we develop a technique to fabricate hierarchical micro/nanostructured surfaces on copper substrates, and exploit the role of microscale roughness elements to increase the droplet departure frequency during the condensation process. To demonstrate the effectiveness of such surfaces, the droplet growth rate at the start of condensation and the cumulative droplet self-removal volume for the hierarchical surface are compared to a surface which features only nanostructures. The hierarchical copper surfaces were fabricated in the Birck Nanotechnology Center at Purdue University. The process fl ow diagram is shown in Figure S1 (Supporting Information). The copper substrate (0.5 mm thick) was fi rst cleaned in a diluted HCl solution (volume ratio HCl: deionized (DI) water = 1:3) for 2 min, rinsed with DI water, and dried with nitrogen. A photoresist layer was then lithographically patterned onto the copper substrate. This process included spin-coating with hexamethyldisilazane (HMDS) at 3000 rpm for 20 s and photoresist AZ 9260 at 1000 rpm for 30 s. Subsequently, the copper was soft-baked at 100 °C for 15 min and exposed for 78 s at a power of 26 mW cm −2 under a chrome mask in a mask aligner (MJB-3, Karl Suss). The mask has square arrays of dots (30 μm diameter) at six different pitches (40, 50, 60, 70, 90, and 105 μm). The exposed photoresist layer was developed using AZ 400K in DI water at a dilution ratio of 1:1.5 for 3 min. A photoresist layer with a thickness of ≈15 μm was produced as a mold for subsequent copper deposition on exposed areas via electroplating. Pulse-electroplating was performed in a custom setup. The electrolyte had three main components: CuSO 4 ·5H 2 O (225 g L −1 ), H 2 SO 4 (40 g L −1 ), and HCl (50 mg L −1 ). The processing parameters (current density, frequency, and duty cycle) were tuned to achieve dense and uniform copper grains within the exposed areas. Electroplating was performed with a current density of 10 mA cm −2 at 1 Hz and 50% duty cycle. The total electroplating time to fi ll the mold was ≈2 h. After electroplating, the copper substrate was soaked in acetone for 2 min to dissolve the AZ 9260 photoresist mold and form copper microposts on the substrate (Figure S2, Supporting Information). Condensation of water vapor is of great interest in thermal management [ 1 ] and power generation [ 2 ] owing to the improved heat transfer by phase change, and in desalination [ 3 ] and dew/ fog harvesting [ 4,5 ] systems for its ability to extract vapor from carrier gases. Enhancement of the condensation heat and mass transfer processes in these systems could lead to considerable performance gains, economic return, and energy savings. Heterogeneous condensation is dramatically infl uenced by the physical structure and chemical properties of a surface. Depending on the surface wettability, water vapor can condense on a surface either as a continuous liquid fi lm (fi lmwise) or as individual droplets (dropwise). It is reported that dropwise condensation can produce heat transfer coeffi cients that are an order of magnitude higher than in fi lmwise condensation. [ 6 ] To attain this performance, condensed droplets must be rapidly removed from the surface and fresh spaces on the substrate exposed for nucleation; otherwise, accumulated large droplets will act as a thermal barrier and inhibit the heat/mass transfer rate. [ 7 ] To promote removal of condensate droplets, the droplet adhesion to the substrate must be minimized. Structured superhydrophobic surfaces [ 8–12 ] could offer an ideal means for enhanced dropwise condensation via improved droplet shedding due to their ability to support spherical water droplets with large contact angle and minimal contact angle hysteresis. However, unlike sessile droplets deposited artifi cially on such surfaces, droplets formed by condensation grow on all interstitial surfaces of the substrate, from the bottom up. Thus, notionally superhydrophobic surfaces (characterized via sessile droplet deposition) do not necessarily preserve their superhydrophobicity during condensation. [ 13–15 ] Previous studies [ 14,15 ] have shown that on superhydrophobic surfaces with only microscale roughness, condensate droplets tend to nucleate and grow in the cavities between microstructures, forming sticky droplets in the Wenzel state, [ 16 ] which are pinned strongly to the surface at the three-phase contact line. To overcome this limitation, superhydrophobic surfaces with nanoscale [ 17,18 ] or hybrid micro/ nanoscale [ 19–24 ] roughness have been developed that enable the formation of condensate droplets in the Cassie state. Although nanostructured surfaces can retain their superhydrophobicity and prevent contact-line pinning of droplets