Rapid Synthesis of Wettability Gradient on Copper for Improved Drop‐Wise Condensation

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.

Vapor-phase crystallization route to oxidized Cu foils in air as anode materials for lithium-ion batteries

“…In the growth of bulk oxide during metal oxidation, the relief of stresses may occur by fracture in the oxide and/or in the underlying metals, or by separation of the oxide-metal interface. 25 In addition, there exists a large volume (y60%) increase accompanied with the conversion of Cu to Cu 2 O. When temperatures were at above 400 uC, copper oxide was easy peeled from the Cu foil owing to geometrically induced stresses that result from volume change.…”

“…At higher reaction temperatures, the diffusion rates of both the O 2 molecules and the Cu atoms are increased, according to Arrhenius-type equation, D = D 0 e 2Q/RT , where D is the diffusivity, D 0 is a proportionality constant independent of temperature, Q is the activation energy of diffusion species, and R is the molar gas constant. 25 More oxygen molecules and the Cu atoms can diffuse to the surface and conduct the oxidation reaction at higher temperature. Thus, the thickness of both Cu 2 O and CuO increases with increase of temperature.…”

“…In addition, the thermal expansion coefficient of Cu 2 O (1.9 6 10 26 K 21 ) is much smaller than that of Cu (17 6 10 26 K 21 ). 25 With the increase of oxidation temperature, the lattice mismatch is reduced and the interfacial strain is decreased, which permits oxygen diffusion to occur more rapidly. Thus the enhanced kinetics of oxygen interfacial diffusion contributes significantly to thicken the copper oxide layers.…”

Fast Water Evaporation from Nanopores

“…Recent development of nano‐science and their applications in solar vapor evaporation [ 11 ] and energy harvesting [ 7,12 ] emphasize the importance of evaporation at nano‐scale. [ 13 ] Insignificant annoyance in micro‐ and macro‐scopic systems could no longer be ignored at nanoscale, for example, Knudsen transport, [ 14 ] disjoining force, [ 15 ] stability under tension, [ 16 ] and hydrophobic effect. [ 17 ] Some of the factors make liquid‐vapor phase change dramatically different in nanoscale and offer valuable inspirations for possible passive evaporation enhancement.…”

Numerical simulation of droplet dynamics on chemically heterogeneous surfaces by lattice Boltzmann method

“…These spontaneous migration phenomena on these biological surfaces have attracted great attention. Thus, the surface structure with wetting gradient has been applied in engineering applications, such as microfluidic devices (Shastry et al , 2006; Lai et al , 2010; Chou et al , 2008; Guo and Tang, 2015), collecting fog (White et al , 2013; Seo et al , 2016) and condensation heat transfer enhancement (Huang et al , 2012). It has been demonstrated that the droplet dynamics on chemically heterogeneous surfaces play an important role in condensation heat transfer enhancement.…”