Study On Superhydrophobicity Of 3D Micro/Nano Surface Structure Based On Gibbs Free Energy

With the development of biomimetic superhydrophobic interface science,the application of biomimetic superhydrophobic surfaces in engineering fields such as green printing and anticorrosion and anti-fouling will be focused on in the national “14th Five-Year Plan” and great long-term strategic needs.The design of controllable wettability and superhydrophobicity solid surfaces has become a hot topic of concern in many disciplines such as mechanics,materials science,biomimetics,and provides new ideas for the development of surface/interface mechanics.The wetting behavior of liquid droplets on solid surfaces is closely related to the inherent surface free energy and surface roughness.At present,the experimental techniques for preparing micro/nanostructured superhydrophobic surfaces are becoming increasingly mature,but the theoretical research on wetting mechanisms is still worthy of further exploration,such as the limitations of projection cut planes in the 2D plane model simulation of microstructure;The issue of differences between 3D models and natural biological surface structures;Quantitative analysis of the complexity of droplet motion through mathematical methods,and so on.Therefore,it is highly promising to break away from the research of phenomenological theory and establish a 3D structural model that conforms to the laws of nature and is suitable for engineering applications to analyze the wetting effect of solid surfaces.Based on Gibbs free energy and thermodynamic analysis methods,this thesis has established four types of 3D models with first-order ordered array cylinder and circular truncate cone structure,dual-scale ordered array cylindrical structure and irregular microstructure.The influence of structural parameters of micro/nanostructure surfaces on the superhydrophobic properties of rough solid surfaces has also been systematically studied.By analyzing the motion trajectory of three-phase contact lines and the pinning rule,the intrinsic relationship between the apparent contact angle and the minimum free energy,as well as the functional relationship between the dynamic contact angle and the free energy barrier have been revealed.In addition,through scanning electron microscope images and wettability measurement experiments on regenerated aspen leaves,the influence mechanism of morphology characteristics and spatial distribution of micro/nanostructures on stable composite state has been elucidated.The research results can provide inspiration and theoretical guidance for the design and preparation of biomimetic superhydrophobic artificial surfaces.The main content of the paper includes the following series of work:(1)Inspired by the principle of “floating on water” of a gecko,a 3D structure thermodynamic analysis model of micro cylinder ordered array is established by simulating its foot bristles.The effects of the intrinsic contact angle,the cylinder base spacing and the cylinder height on the free energy and the free energy barrier of the composite and non-composite states are studied.The transition law of the wetting state of the system with three geometric parameters as variables is discussed separately.Three expressions of the critical intrinsic contact angle,the critical distance and the critical height are proposed,the feasibility mechanism of controllable high surface roughness for obtaining composite state is revealed.(2)Expansion of cylinder structures to their general deformation-the circular truncated cone structure,and taking into account the possible size effects of surface roughness in submicroscopic dimensions,a 3D structure model of an ordered array of circular cones is established to simulate the motion trajectory of three-phase contact lines under different wetting conditions.Discovered a positive correlation between the energy accumulation of three-phase contact lines and the equilibrium contact angle in the composite state.In addition,the theoretical analysis of the relationship between the base angle and base spacing of the circular truncated cone and the wetting behavior is conducted,and its critical expression is derived.It is clarified that the morphological parameters of the circular truncated cone structure played a key role in the transition between two typical wetting states.(3)Based on the “lotus leaf effect” biomimetic structure,a dual-scale cylindrical ordered array layered micro/nanostructure model is proposed,which defines the system as four wetting states at the micro and nano scales,and establish corresponding transient equations.The critical values and calculation formulas for the transition of four wetting states are obtained by reasonably adjusting the height and spacing of nanocylinders.It is found that the two scale geometric parameters coordinated to adjust the global wetting state,revealing that the impact of nanostructures on free energy barriers is the intrinsic reason for improving the superhydrophobicity of surfaces.This method has a certain guiding significance for the preparation of various types of superhydrophobic surfaces,especially for strongly adhesive superhydrophobic surfaces(petal effect).(4)Based on the disordered structure of the compound eye surface of sawwing moth,a 3D irregular microstructure rough surface is proposed.The surface contains six different protrusions(cone,cylinder,truncated cone,inverted truncated cone,paraboloid and spherical crown),which are randomly arranged in a hexagonal lattice(similar to honeycomb structure).The results show that the wetting state largely depends on the actual distribution of various shapes,and changing the ratio of the six protrusions will affect the surface free energy or free energy barrier.In addition,a spontaneous transition mechanism of the wetting state of the system is discovered,and a critical transition expression is provided for non-spontaneous energy transitions.This method is suitable for analyzing the effects of structural disturbances observed in real biological surfaces on the wetting state,and can reasonably predict the equilibrium and dynamic contact angles of the system.(5)Based on the growth mechanism of leaf regeneration after damage to aspen trees,the correlation between the morphological characteristics of the microstructure of the regenerated leaves and the wetting behavior of the leaf surface are analyzed.Through scanning electron microscope images,it is found that the regenerated leaves have a dual-scale structure composed of nano epidermal crystal wax superimposed on micron papillae,revealing the positive impact of hemispherical microstructure and disorderly arranged nanocrystal structures on activating surface superhydrophobicity and maintaining stable composite state.This discovery lays an experimental foundation for designing a spatial structure model with dual-scale irregularities to enhance the robustness of superhydrophobic surfaces.