Surface Wettability of Macroporous Anodized Aluminum Oxide

Abstract

The correlation between the structural characteristics and the wetting of anodized aluminum oxide (AAO) surfaces with large pore sizes (>100 nm) is discussed. The roughness-induced wettability is systematically examined for oxide films grown by a two-step, high-field anodization in phosphoric acid of three different concentrations using a commercial aluminum alloy. This is done for the as-synthesized AAO layers, after various degrees of pore widening by a wet chemical etching in phosphoric acid solution, and upon surface modification by either Lauric acid or a silane. The as-grown AAO films feature structurally disordered pore architectures with average pore openings in the range 140-190 nm but with similar interpore distances of about 405 nm. The formation of such AAO structures induces a transition from slightly hydrophilic to moderately hydrophobic surfaces up to film thicknesses of about 6 μm. Increased hydrophobicity is obtained by pore opening and a maximum value of the water contact angle (WCA) of about 128° is measured for AAO arrays with a surface porosity close to 60%. Higher surface porosity by prolonged wet chemical etching leads to a rapid decrease in the WCA as a result of the limited pore wall thickness and partial collapse of the dead-end pore structures. Modification of the AAO surfaces by Lauric acid results in 5-30° higher WCA's, whereas near-superhydrophobicity (WCA ~146°) is realized through silane coating. The "rose petal effect" of strongly hydrophobic wetting with high adhesive force on the produced AAO surfaces is explained by a partial penetration of water through capillary action into the dead-end pore cavities which leads to a wetting state in-between the Wenzel and Cassie states. Moreover, practical guidelines for the synthesis of rough, highly porous AAO structures with controlled wettability are provided and the possibility of forming superhydrophobic surfaces is evaluated.

Morphology-influenced wetting model of nanopore structures

“…The second divergence is between the simulation results as calculated with the inclusion of correction factors and the actual experimental results. Surface energy variation, a result of oxidation effects or surface irregularities, is likely the primary reason for the discrepancies between the simulation results and the experimental results [21,23].…”

“…Such surface irregularities can increase the uncertainty of the simulation results. Figure 5 shows the modeling and experimental results for the contact angles on alumina nanopore structures (γ = 0.072 N/m), based on data from the study published by Buijnsters et al [21]. As the figure shows, the discrepancy between the simulation and experimental results is not significant.…”

Bacteria repelling on highly-ordered alumina-nanopore structures

“…These results correlate with published data. 37 Area percentage of adherent bacterial cells on ANS Table II shows the statistical data results for adherent bacterial cells on the ANSs. Figure 2 shows the represented optical microscope images (color converted: black and white) of bacterial cells on ANS.…”

Effects of Electrolyte and Ti Layers on Static and Dynamic Friction of Anodized Alumina

“…Improved hardness, anti-corrosion, paintability and other properties encourage the application of anodized alumina in aviation, electronics, automotive, food, marine industries and even in medicine. Hard anodizing ("Type III") carried out in sulfuric acid-based electrolytes typically leads to thick Al2O3 coatings with nanopore diameters less than 20 nm, whereas phosphoric acid-based electrolytes may produce coatings with large pore diameters exceeding 100 nm or more [2]. Despite better hardness, friction and wear rate of anodized Al2O3 coatings is still very high, often above Coefficient of Friction (COF) of 1.0 [3].…”