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Abstract

We describe a simple method of making wettability gradients for the first time on copper using temperature controlled surface oxidation. The temperatures of the sample copper sheet are controlled to change from 330 to 620 K for a distance of 36 mm, with the presence of oxygen (air). Cone-like Cu2O nanostructures are grown on the sample surface due to surface oxidation. Since the rate of oxidation varies with temperatures, the heights of the produced nanocones increase gradually from the cold end to the hot end, which create a wettability gradient in between, with water contact angles changing from 88° (the cold end) to 18° (the hot end). The same method can also be used for copper wires and tubes. Due to the extensive use of metal materials in industry, the technique presented here may have great importance for practical applications. The produced wettability gradient on copper, as an example, is demonstrated to be able to enhance water condensation rate up to 30%, depending on the direction of the gradient relative to gravity.