Session: 09-03: Computational Methods in Micro/ Nanoscale Transport

Paper Number: 131175

131175 - Molecular Dynamics Investigation of Wettability Transition of Copper and the Design of Durable Superwetting Structures 

Abstract: 

Research on metals such as copper, aluminum, and gold suggests that their clean surfaces exhibit strong hydrophilic characteristics (superwettability) due to high surface energy. However, in natural environments, these metal surfaces often appear moderately hydrophobic. What causes the wettability transition of these metals? We propose that the absorption of airborne organics plays a significant role based on the following observations: carbon chains in organic compounds exhibit water-repellent properties, organics readily adhere to metal surfaces due to attractive interactions with metal atoms, and airborne contaminants comprise a substantial portion of organics.
To verify this perspective, Molecular Dynamics simulations were conducted. The simulation domain constitutes a copper slab with two cleavage surfaces exposed to a water environment. In the absence of organics, the surface energy of copper obtained from the simulation (1.79 J/m^2) closely matched experimental measurements (1.70 J/m^2), indicating high hydrophilicity of the clean copper surface. However, upon introducing organic acid molecules (e.g., caprylic acid) into the system, the calculation of surface energy became more complex due to the irregularity of the copper surface formed by copper atoms and absorbed organic molecules. To simplify the calculation we utilized the surface energy of water to observe the wettability transition. In the presence of pure copper atoms, the surface energy of water was -0.11 J/m^2, signifying strong hydrophilicity. As the coverage ratio of caprylic acid increased, the surface energy of water also increased, indicating reduced hydrophilicity. The wettability transition was observed when the coverage ratio exceeded 0.47, and the surface energy became positive, suggesting a hydrophobic nature. Upon complete coverage of the copper surface by caprylic acid molecules, the surface energy of water reached approximately 0.08 J/m^2. Further increasing the number of acid molecules did not significantly affect the surface energy, as excess molecules accumulated and formed a stable thick layer on the substrate.
The wettability transition rate was found to be influenced by the length of the carbon chain in organics. When a shorter carbon chain organic acid, such as acetic acid, was absorbed, the increasing slope of the surface energy was gentler. Additionally, these molecules exhibited higher likelihood of leaving the metal surface and dissolving into the water, particularly when the coverage ratio exceeded 0.5. Due to this instability, the maximum surface energy reached was only 0.03 J/m^2, representing a 63% decrease compared to the surface covered by caprylic acid molecules.
Based on these findings, we propose a surface structure that can maintain durable superwettability: a nanoporous surface with sufficiently small voids that allow water molecules to enter while preventing the penetration of organic molecules. This surface structure is achieved through an in-situ laser deposition method, where metal materials undergo rapid heating and subsequent phase explosion under picosecond laser pulses. The resulting ablated products primarily consist of fine nanoparticles, which rapidly expand and cool in ambient air, redepositing near the laser-irradiated areas to form a dense nanoporous layer. These nanoporous layers comprise numerous fine nanoclusters with structural gaps smaller than 5 nm. The prepared surfaces exhibit exceptional long-lasting superwettability for over two years in laboratory atmosphere. Such surfaces hold significant potential for applications involving phase change heat transfer and chemical reactions at solid-liquid interfaces.

Presenting Author: Zhigang Huang Guangdong University of Technology

Presenting Author Biography: Professor Zhigang Huang is a Fellow of the Chinese Society of Mechanical Engineering. He studied in the School of Electro-Mechanical Engineering of Guangdong University of Technology and obtained his PhD in 2007. He conducted postdoctoral research at the Hong Kong Polytechnic University from 2007 to 2009. He subsequently worked for Guangdong University of Technology and became a full professor in 2013. Prof. Huang's research primarily focuses on micro and multi-scale simulation methods, with a specific emphasis on their application in the fields of microfluids and microstructure mechanics.

Authors:

Zhigang Huang Guangdong University of Technology
Jiangyou Long Guangdong University of Technology
Kai Luo University College London