Study of Micro- and Nanopatterned Aluminum Surfaces Using Different Microfabrication Processes for Water Management

Abstract

Superhydrophobic surfaces demonstrate extreme water-repellence, promoting drop-wise over film-wise condensation, increasing liquid mobility, and reducing thermal resistance for heat-exchanger applications. Introducing topographic structures can lead to modified surface free energy, as inspired by natural systems like the lotus leaf, potentially allowing coating-free ice- and frost-free surfaces under certain conditions. This work presents a study of coating-free aluminum micro/nanopatterns fabricated using micromilling or laser-etching techniques and the resultant wetting properties. Our review and experiments clarify the roles of line-edge-roughness and microstructural geometry from each microfabrication technique, which manifests in technique-specific nano- to midmicro-scale roughness, producing a hierarchical structure in both cases. For micromilling, line-edge-roughness consists of jagged burrs of 1–8 μm thickness with 10–25 μm periodicity along the microlines with constantly changing height on the order of 1–20 μm. These effects simultaneously raise the water contact angle from 52° (unprocessed aluminum) up to 136° but with strong edge pinning effects. On the other hand, laser-etched surfaces exhibit line-edge-roughness with a microstructure of 3–20 μm width and 5–10 μm in height superimposed with evenly spread spikes of 50–250 nm. This results in a high contact angle (>150°) coupled with a low contact angle hysteresis (<15°), promoting superhydrophobicity on a coating-free aluminum surface. It is also shown that for certain cases, line-edge-roughness is more important for the resultant wetting properties than the structure geometry.

Development of a Coating-Less Aluminum Superhydrophobic Gradient for Spontaneous Water Droplet Motion Using One-Step Laser-Ablation

“…That combination is important because of the complicated nature of wetting. Even while having a high static CA of around 150° or higher, the surface might have CAH of the same order, which is called the rose-petal effect, leading to very limited droplet mobility as has been observed on processed aluminum …”

“…Existing methods of mechanical processing (e.g., micromilling) and laser treatment (as direct laser writing and direct laser interference patterning) allow a high hydrophobic (HPB) state (static contact angle (SCA) in the range of 140–148° , ) to be obtained or even an super-hydrophobic (SHPB) state with an SCA of around 155–160° and a relatively high contact angle hysteresis (CAH) around 30–50°. − Using nanosecond lasers it is possible to reduce the CAH to ≤15° by controlling surface roughness, which correlates with the commonly used range of superhydrophobicity in the literature (SCA > 150°; CAH <15°). ,,, …”

“…Another important note related to the particular topography modification is that any anisotropic modifications (e.g., grooves) cause anisotropic wetting, where the apparent CA is dependent on the viewing direction, which is more relevant and a lot more noticeable for the Wenzel state Figure g,h shows an anisotropic surface (i.e., the structured surfaces) with characteristic elongation of a surface droplet leading to different apparent contact angles along perpendicular directions.…”

“…In the case of one-step microfabrication (which is the focus of this work), a microstructure is created via a set of microchannels. Considering that the droplet is always lifted by the top of the microchannels and their edges and that the channels are always filled with air, the area fraction f might be evaluated as the ratio of the channel peak width to the pitch of the whole structure . The best reported case in ref with that assumption provides f = 0.1, which will be used here as a benchmark for further calculations.…”

On-Demand Fully Enclosed Superhydrophobic–Optofluidic Devices Enabled by Microstereolithography

“…Periodic microgratings and micropins are shown in Figures a and b, respectively, with aspect ratios of ∼5.7 and 8, respectively. These periodic structures have been widely used for various superhydrophobicity applications. ,, Figure c shows a “T-shaped” double overhang design with super-repellent capability, since the low contact angle prevents the liquid from entering the air gap . A slightly modified “umbrella” design is shown in Figure d, which changes the flat “T-shaped” cap to a 13.31° slope.…”