Wetting behavior and mechanisms of rough metallic surfaces
Creators: name: JIANG, Yue
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401607
subject: Metals and alloy materials

description: Wettability is one of the most important properties of rough solid surfaces. In this PhD thesis, a thermodynamic analytical model was firstly developed to determine the wetting equilibrium and describe the wetting behavior and properties of rough surfaces with arbitrary surface structures. Based on this model, generalized theoretical expressions of the apparent contact angles (CAs) in the Cassie-Baxter (CB) state and the Wenzel (W) state, the threshold Young angle of the metastable CB state, the energy barrier separating the metastable CB and W states, and the critical pressure preventing the wetting transition from the CB to W state were derived.

Secondly, by applying the generalized expressions to four types of surfaces built with protrusions in paraboloid, truncated cone, inverted truncated cone and flat-top pillar shapes, the equilibrium wetting states of these surfaces were determined, and the corresponding expressions of the threshold Young angle, energy barrier and critical pressure were given. The effects of the surface structure and surface chemical compositions on the wetting behavior, the meanings of the threshold Young angle of the metastable CB state, as well as the roles of the energy barrier and the critical pressure in preventing the wetting transition of these surfaces were elaborated. A general guidance on the optimization of the surface structure and Young angle value of the substrate material for achieving robust superhydrophobicity on the studied surfaces was then proposed. 

Thirdly, by using the electric brush-plating and laser ablation techniques, respectively, the copper (Cu) and brass surfaces with lotus-like, three-level, micro/nano hierarchical structures were fabricated. By performing the direct and cyclic heating treatments, measuring the overall wetting properties, and analyzing the surface chemical compositions, the relationships of the wetting properties, heat treatment parameters, surface structures, and surface chemical compositions of these two types of surfaces were established.

Finally, the mechanisms of the surface hierarchical structures in leading to the wettability transition from superhydrophilicity to superhydrophobicity (WTSS) of the electric brush-plated Cu surface and the reversible WTSS of the laser-ablated brass surface were analyzed.
The key findings of the thesis can be summarized as follows:
The threshold Young angle is an important parameter determining the variation tendency in the Gibbs energy of a system. The the threshold Young angle can be used to determine the equilibrium wetting state of a rough surface according to its surface structure and Young angle. According to the critical Young angle or the equilibrium Gibbs energies in the CB and W states, the CB state can be further divided into the metastable CB state, and the stable CB state. In these two states, the CB-W transition can still take place once the energy barrier or the critical pressure is overcome by external stimuli. 

The energy barrier originates from the contributions of the replacement energy, and the liquid-air interfacial energy change. The critical pressure originates from the contributions of the replacement energy gradient, and the liquid-air interfacial energy gradient. Different dependence of the energy barrier and the critical pressure components on surface structure and Young angle provide a guidance to achieve high stability of the CB state of different surfaces. 

The critical pressure is independent on the droplet volume, while the energy barrier depends on it. The critical pressure is related to the absolute scale of the surface geometrical parameter D, while the energy barrier is related to the ratios of the surface geometrical parameters P/D and H/D. The critical pressure is more sensitive to the geometrical shape of the protrusion than the critical pressure. These features of critical pressure and energy barrier suggest that the critical pressure instead of the energy barrier should be used as a criterion to quantitatively evaluate the stability of the CB state of rough surfaces. 

For the four studied surfaces built with paraboloid, truncated cone, inverted truncated cone and flat-top pillar-shaped protrusions, an appropriate P/D value should be used to obtain superhydrophobic surfaces (SHSs) with the equilibrium apparent contact angle and critical pressure values. For the parabolic surface, a larger H/D value should be used to achieve SHSs with both higher apparent contact angle and critical pressure values. For the truncated cone, inversed truncated cone and flat-top pillar surfaces, a larger H/D value, a smaller H/D value and a smaller D value should be used respectively to obtain SHSs with higher critical pressure values. At a relatively higher level of apparent contact angle, the paraboloid surface shows the strongest ability to achieve SHS with a higher critical pressure value; at a relatively lower level of apparent contact angle, the inversed truncated cone surface exhibits a stronger ability to achieve SHSs with a higher critical pressure value even if its substrate material is hydrophilic.

The topological characteristics, including the size and distribution of the micro-scale protrusions and submicro-scale bumps on the electric brush-plated Cu surfaces are similar to those observed on the natural lotus leaf. The surface hierarchical structure controls the wetting properties and leads to the WTSS and the reversible WTSS by producing a two-tier magnifying effect on the wettability of the substrate material. In addition to significant changes in the apparent contact angles in the CB and W states, another important finding of this study is that the two-tier magnifying effect significantly changes the threshold Young angles in the CB and W regions which leads to a great reduction in the width of the W region. It is this reduction in the width of the W region that makes the WTSS and the reversible WTSS become possible when the Young angle of the nano-structured substrate is changed in a small range. 

Controlling the adsorption and decomposition of airborne organic compounds through heat treatment is an effective method to control the wetting properties of rough metallic surfaces. The change in the wetting properties arises mainly from the contribution of the content of the nonpolar C¿C/C¿H species. Since the adsorption and decomposition processes can occur effectively at lower temperatures and short time, the heat treatments do not lead to a significant change in the surface structures, and the wetting properties of the surfaces can remain stable and reversible. 

The reversible WTSS can be achieved on the laser-ablated brass surfaces by using alternate heating-reheating cycles. This reversible WTSS arises from the reversible change in the Young angle value and the magnifying effect of the hierarchical structure. The superhydrophobic laser-ablated surface possesses low surface adhesion and potential self-cleaning ability, and also shows high stability in long-term room temperature storage and good durability underwater.