Effects Of Multiscale Micro/Nano Structured Surfaces On Superhydrophobic Property

Superhydrophobicity of biological surfaces has recently been studied intensively with the aim to design ideal artificial surfaces. It has been revealed that nearly all the superhydrophobic surfaces consist of the intrinsic hierarchical structures. However, the effects of different geometrical parameters for a multi-scale of hierarchical structure on its wettability and the physical mechanism have not been completely understood. In particular, there are hardly any theoretical studies to investigate quantitatively the thermodynamic mechanisms for the unique role of such hierarchical structures in superhydrophobic behavior.In this study, different scales hierarchical structures have been thermodynamically analyzed using a 2-D model. The free energy (FE) and free energy barrier (FEB) for the composite wetting states of different scales hierarchical structures are calculated respectively, and the effects of different scales hierarchical structures, relative pillar height (hr), relative pillar width (ar) on contact angle (CA) and contact angle hysteresis (CAH) have been investigated in detail. Finally, equilibrium contact angle, advancing angle, receding angle and its hysteresis for various orientations of the dual- roughness microstructure are calculated based on the proposed thermodynamic model. We discuss anisotropic wetting behavior for dual-roughness structure and the effects of vibration energy. The main contents and results are followed as:1. Different scales hierarchical structures have been thermodynamically analyzed using a 2-D model. The results show that from different enlarging curves of FEB, we can see that the nano- to micro- structures of such surfaces can split a large FEB into many small ones and hence can decrease FEB. For extremely small droplets, the secondary or third structure (i.e., submicrostructure or nanostructure) can play a dominant role in resisting the droplets into troughs, so that a composite state can be always thermodynamically favorable for such hierarchical structured system.2. Taking dual- scale roughness structure as the example, we discuss the effects of relative height and relative pillar width on ECA, wetting transition and FEB. The calculation shows that larger relative height (above 0.75) leads more stable composite wetting system; smaller relative pillar width (below 1) brings larger ECA and smaller CAH. 3. The effects of different scales hierarchical structures on contact angle and contact angle hysteresis have been investigated. When the geometrical parameter ratio for different scales of hierarchical structure is the same, it is found that the three- scale roughness structure has the largest ECA and lowest receding free energy barrier.4. Anisotropic wetting behavior for dual-roughness structure and the effects of vibration energy are discussed. It is revealed that the strong anisotropy of equilibrium contact angle (ECA) and contact angle hysteresis (CAH) is shown in the noncomposite state but almost isotropic wetting properties are exhibited in the composite state. When the vibration energy received from external sources increases, the contact angle hysteresises for both wetting systems and the anisotropy of contact angle hysteresis for noncomposite wetting system decrease, but the anisotropy of contact angle hysteresis for composite wetting system increases.