An oil droplet in water can be in the Cassie state (with water and/or air trapped between the solid and oil) with a high contact angle (top left) or in the Wenzel state (top right). Depending on the roughness of the brass substrate, both states with high (bottom left) and low (bottom right) contact angle are observed.

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The high temperature oxidation of Cu-32.02% Zn-2.30%Pb brass was carried in N 2 -5wt.% O 2 and N 2 -12 wt.% O 2 atmospheres. The amounts of oxygen in the oxidizing atmospheres and the time of the oxidation affected the oxide morphologies and kinetics of the oxide growth. In the first hour of the oxidation at 650 °C, oxide nanowires were noted. The average diameter, length and distance between the observed nanowires were 27 ± 0.01 nm, 0.20 ± 0.04 µm and 0.20 ± 0.04 µm respectively for the samples oxidized in N 2 -5wt.% O 2 atmosphere and 102 ± 23 nm, 0.36 ± 0.24 μm and 0.24 ± 0.08 μm respectively for the samples oxidized in N 2 -12wt.% O 2 atmosphere. The EDX and XRD analyses of the nanowires and the oxide granules confirmed ZnO nanowires and a continuous oxide layer of ZnO. The x-ray diffraction confirmed minor presence of PbO. The oxide growth kinetics followed the linear oxide growth model, for the alloy samples that were thermally oxidized in N 2 -5 wt.% O 2 atmposphere and parabolic growth model for those thermally oxidized in N 2 -12 wt.% O 2 atmospheres respectively. The values of 6.8 µm/hour and 23.03 µm/(hour) 1/2 were determined for growth constant (k), based on the two models respectively.

Abstract:The underwater contact angle behavior on oxide layers of varying thicknesses was studied. These oxide layers were grown by thermally oxidizing C84400 copper alloys in N 2 -0.75 wt.% O 2 and N 2 -5 wt.% O 2 gas mixtures at 650 °C. Characterization of the oxidized specimens was effected using X-ray diffraction, scanning electron microscope (SEM) and contact angle goniometer. The results from the X-ray diffraction analyses confirmed the formation of CuO, ZnO and PbO. The average sizes of the oxide granules were in the range of 70 nm to 750 nm, with the average thickness of the oxide layer increasing with the increase in the weight percent of oxygen in the N 2 -O 2 gas mixtures. The results showed that the oxide layer growth followed the parabolic law. The underwater oil contact angles increased, due to the change in the surface morphology and porosity of the oxide layer. The small sizes and irregular packing of the oxide granules cause hierarchical rough surface layers with pores. The estimated pore sizes, in the range of 88 ± 40 to 280 ± 76, were predominant on the oxide layers of the samples processed in the N 2 -5 wt.% O 2 gas mixture. The presence of these pores caused an increase in the porosities as the thickness of the oxide layers increased. At oxide layer thickness above 25 microns, the measured contact angle exceeded 150° as underwater superoleophobicity was recorded.

The photodegradation of stearic acid has been studied through evaluation of changes in the contact angles of water and from absorbance measurements. The photodegradation of 0.02 M stearic acid coatings and solutions were initiated by TiO 2 nanoparticles of average size of 9.80 ± 2.92 nm embedded in cements in 1.66 wt.%, 3.33 wt.%, 5.0 wt.% and 6.67 wt.% to generate modified cement composites with photocatalytic capability. It was noted that the photodegradation efficiencies increased with the increase in the weight-percent of TiO 2 present in the modified cement samples. A modified Cassie-Baxter and the Langmuir-Hinselwood models were used to compute the rate constants, based on changes in the contact angles of water and in the concentration of the stearic acid respectively, on exposure to the UV light source. The modified Cassie-Baxter model successfully provided a route to relate the changes in water contact angle to the rate of photodegradation of a hydrophobic, long-chain stearic acid. The values of the rate constant estimated from both models increased with increase in the amount of TiO 2 present in the modified cement samples. However, the rate constant values obtained from the modified Cassie-Baxter model were lower than those obtained from the Langmuir-Hinselwood model. The values of these rate contants were in the range of 0.11-0.50 hr -1 and 0.78-1.33 hr -1 as btained from the modified Cassie-Baxter and Langmuir-Hinselwood models respectively. This disparity in the values was attributed to a higher mobility of the charge carriers and free-radicals that induced the photodegradation in liquid medium as compared to the solid medium.