Fabrication Optimization of Ultra-Scalable Nanostructured Aluminum-Alloy Surfaces

Aluminum and its alloys are widely used in various industries. Aluminum plays an important role in heat transfer applications, where enhancing the overall system performance through surface nanostructuring is achieved. Combining optimized nanostructures with a conformal hydrophobic coating leads to superhydrophobicity, which enables coalescence induced droplet jumping, enhanced condensation heat transfer, and delayed frosting. Hence, the development of a rapid, energy-efficient, and highly scalable fabrication method for rendering aluminum superhydrophobic is crucial. Here, we employ a simple, ultrascalable fabrication method to create boehmite nanostructures on aluminum. We systematically explore the influence of fabrication conditions such as water immersion time and immersion temperature, on the created nanostructure morphology and resultant nanostructure length scale. We achieved optimized structures and fabrication procedures for best droplet jumping performance as measured by total manufacturing energy utilization, fabrication time, and total cost. The wettability of the nanostructures was studied using the modified Cassie-Baxter model. To better differentiate performance of the fabricated superhydrophobic surfaces, we quantify the role of the nanostructure morphology to corresponding condensation and antifrosting performance through study of droplet jumping behavior and frost propagation dynamics. The effect of aluminum substrate composition (alloy) on wettability, condensation and antifrosting performance was investigated, providing important directions for proper substrate selection. Our findings indicate that the presence of trace alloying elements play a previously unobserved and important role on wettability, condensation, and frosting behavior via the inclusion of defect sites on the surface that are difficult to remove and act as pinning locations to increase liquid-solid adhesion. Our work provides optimization strategies for the fabrication of ultrascalable aluminum and aluminum alloy superhydrophobic surfaces for a variety of applications.

Stable Dropwise Condensation of Ethanol and Hexane on Rationally Designed Ultrascalable Nanostructured Lubricant-Infused Surfaces

Vapor condensation is a widely used industrial process for transferring heat and separating fluids. Despite progress in developing low surface energy hydrophobic and micro/nanostructured superhydrophobic coatings to enhance water vapor condensation, demonstration of stable dropwise condensation of low-surface-tension fluids has not been achieved. Here, we develop rationally designed nanoengineered lubricant-infused surfaces (LISs) having ultralow contact angle hysteresis (<3°) for stable dropwise condensation of ethanol (γ ≈ 23 mN/m) and hexane (γ ≈ 19 mN/m). Using a combination of optical imaging and rigorous heat transfer measurements in a controlled environmental chamber free from noncondensable gases (<4 Pa), we characterize the condensation behavior of ethanol and hexane on ultrascalable nanostructured CuO surfaces impregnated with fluorinated lubricants having varying viscosities (0.496 < μ < 5.216 Pa·s) and chemical structures (branched versus linear, Krytox and Fomblin). We demonstrate stable dropwise condensation of ethanol and hexane on LISs impregnated with Krytox 1525, attaining about 200% enhancement in condensation heat transfer coefficient for both fluids compared to filmwise condensation on hydrophobic surfaces. In contrast to previous studies, we use 7 h of steady dropwise condensation experiments to demonstrate the importance of rational lubricant selection to minimize lubricant drainage and maximize LIS durability. This work not only demonstrates an avenue to achieving stable dropwise condensation of ethanol and hexane, it develops the fundamental design principles for creating durable LISs for enhanced condensation heat transfer of low-surface-tension fluids.

Effect of Latent Heat Released by Freezing Droplets during Frost Wave Propagation

Frost spreads on nonwetting surfaces during condensation frosting via an interdroplet frost wave. When a supercooled condensate water droplet freezes on a hydrophobic or superhydrophobic surface, neighboring droplets still in the liquid phase begin to evaporate. Two possible mechanisms govern the evaporation of neighboring water droplets: (1) The difference in saturation pressure of the water vapor surrounding the liquid and frozen droplets induces a vapor pressure gradient, and (2) the latent heat released by freezing droplets locally heats the substrate, leading to evaporation of nearby droplets. The relative significance of these two mechanisms is still not understood. Here, we study the significance of the latent heat released into the substrate by freezing droplets, and its effect on adjacent droplet evaporation, by studying the dynamics of individual water droplet freezing on aluminum-, copper-, and glass-based hydrophobic and superhydrophobic surfaces. The latent heat flux released into the substrate was calculated from the measured droplet sizes and the respective freezing times ( t_f), defined as the time from initial ice nucleation within the droplet to complete droplet freezing. To probe the effect of latent heat release, we performed three-dimensional transient finite element simulations showing that the transfer of latent heat to neighboring droplets is insignificant and accounts for a negligible fraction of evaporation during microscale frost wave propagation. Furthermore, we studied the effect of substrate thermal conductivity on the transfer of latent heat transfer to neighboring droplets by investigating the velocity of ice bridge formation. The velocity of the ice bridge was independent of the substrate thermal conductivity, indicating that adjacent droplet evaporation during condensation frosting is governed solely by vapor pressure gradients. This study not only provides key insights into the individual droplet freezing process but also elucidates the negligible role of latent heat released into the substrate during frost wave propagation.