Optimal Design Of Superhydrophobic Geometrical Surfaces Based On Thermodynamic Analysis

Superhydrophobic surfaces with large contact angle (CA) and small contact angle hysteresis (CAH) have been gained much intense interest from the academic aspect due to their ideal liquid-shedding or droplet-sliding properties over the past two decades. Such surfaces with unique wettability property have shown wide promising applications, such as microfluidics, bio-chips, satellite antenna dust, cable anti-ice and so on. It is studied that solid surface hydrophobicity is depended on the surface microstructure and surface chemical composition. However, it is limited to enhance hydrophobicity by a chemical modification of low the surface energy, the micro-geometric surface structures are obvious for superhydrophobicity, especially for equilibrium contact angle (ECA), as micro-geometric surface greatly increases the roughness ratio. Due to the complication of the relation between the contact angle of roughness surface and wettbility, it is not clear that the relation between the physical mechanism and wettbility so far. In addition, the three-phase wetting system with solid-liquid-air is affected seriously by external environment such as vibrational energy. Therefore, it is very important to theoretically study superhydrophobicity, especially the contact angle hysteresis more than contact angle, and the relation between them and micro-textured structured surfaces. For the innovation of this paper, we used a superhydrophobic thermodynamic model to establish the relation between CA and free energy (FE) and between CAH and free energy barrier (FEB), and to systemically analyze the relation among micro-structured geometrical surface, intrinsic CA with wettability, especially the directly relation with CAH. The main contents are followed as:1. By the thermodynamic analysis of trapezoid micro-structured surface, we discussed the effects of geometrical parameters on ECA, CAH and wetting transition. It is found that the transition between noncomposite and composite states can happen only both high and base angle reaching the critical level; moreover, the stable composite superhydrophobic surface can be form. With the decreasing of base-width, the ECA increases and the CAH decreases, for the base-width of zero, the advancing CA equals to the receding CA and equals to the ECA, and the CAH is zero, especially the ECA for the composite state is as high as 176°, the micro-textured surface (i.e., sawtooth structure) has an ideal superhydrophobicity. For the base-spacing of zero, the ECA of composite state decreases from 151°to 138.6°.2. For the effects of intrinsic CA on superhydrophobic surface, it is found that hydrophilic surface (e.g.,θ_Y = 80°) is more unstable than hydrophobic surface (e.g.,θ_Y =120°). For the hydrophilic surface, the noncomposite state is more stable than composite state, moreover, there is no wetting transition. However, for the hydrophobic surface, the composite state is more stable than noncomposite state, moreover, there is wetting transition.3. For the effects of FEB on CAH, it is found that there is negative FEB for low intrinsic CA surface (i.e., hydrophilic surface), if the wetting system receives FE from external sources to overcome FEB, the advancing CA increases, the receding CA decreases and the CAH increases. However, for high intrinsic CA surface (i.e., hydrophobic surface), there is positive FEB, if the wetting system receives FE from external sources to overcome FEB, the advancing CA decreases, the receding CA increases and the CAH decreases.