Thermodynamic Analysis On The Wetting Behavior Of Solid Surfaces Based On Gibbs Free Energy

The wetting behavior of hydrophobic solid surfaces has attracted significant attention due to their potential applications in practical life.Theoretical analysis on the wetting behavior of solid surfaces not only benefits the manufacture and development of biomimetic materials,but also promotes the development of Surface Mechanics.The wetting behavior of hydrophobic surfaces mainly depends on the surface roughness structures and chemical composition.In the aspect of theoretical modeling,the relationship between free energy/free energy barrier and surface configuration has been systematically investigated.However,the existing models either involve complicated calculations,or are based on a simplified two-dimension model.It is of great research value to propose a more sophisticated model for the wetting behavior of hydrophobic surfaces.The relationship between the movement of three-phase contact line and the free energy changes during the advancing/receding process of the droplet is established based on the actual shape of the three-phase contact line,a three-dimension model combing the apparent contact angle and the minimum free energy,dynamic contact angles and free energy barriers has been proposed in this paper for analyzing the hydrophobicity of solid surfaceWith the assistance of the proposed 3-D model and Surface Evolver,the effect of the roughness structure on superhydrophobicity has been investigated,the wetting behaviors of the solid surfaces with single-scale pillar textured surface,two-scale pillar textured surface,striped surface have been analyzed in detail,the obtained results provide guidance for fabricating hydrophobic surfaces.The main content of this thesis is listed as follows:(1)The surface with square-pillar texture is used as an example to show the 3-D model,the effect of pillar height on the free energy and free energy barrier for different wetting states is investigated,the effect of pillar height on the transition between the two wetting states is also analyzed,the result shows that the composite state is more stable if the pillar height is higher than the critical pillar height,and the system prefers the noncomposite state otherwise.The formula for determining the critical pillar height is deduced,and the application of the model is extended to the pillar textured surfaces with arbitrary cross-sections(2)Inspired by the Lotus effect and the petal effect,the Cassie state,Cassie Impregnating wetting state,and Wenzel state of the surface with two-scale roughness are analyzed.The results indicate that the Nano pillar height also plays a crucial role in the transition between each two wetting states.Finally,a schematic diagram is presented to determine the final stable wetting state.(3)Size effect exists when the size of roughness structure shrinks into the micron-submicron range.The three-phase contact line tension is introduced to analyze the wetting behavior of solid surfaces with trapezoidal microarchitecture.The results show that the accumulated total line energy becomes significant as the roughness scale shrinks into the micron-submicron range,and subsequently increases the apparent contact angle.Moreover,the effect of trapezoid base angle and spacing on the wetting behavior is investigated,several suggestions are made for fabricating the hydrophobic surface with trapezoidal microarchitecture based on the obtained results.(4)A three-dimension model is proposed for analyzing the anisotropic wetting with the assistance of Surface Evolver,the variations of free energy and strip number during the spreading process are analyzed,and the result shows that small wet stripe width or large dry stripe width is necessary to achieve larger apparent contact angle and smaller contact angle hysteresis.In addition,the wetting stability of anisotropic wetting is investigated by changing the droplet volume,the slip-jump behavior is observed,and another method to analyze the dynamic contact angles of anisotropic wetting is proposed.The theoretical prediction of the dynamic contact angles of anisotropic wetting can be narrowed down by combing the two methods.