Influences By Surface Microscopic Characteristics On Droplet Wetting State And Stability Of Gas-liquid Interface

The evaporator heat transfer surface design is one of the important applications of the wetting phenomenon.Among evaporator producing and operating processes,the roughness and heterogeneity of the surface arised by manufactured technological level,the anisotropy caused by dirt or crystallization,and the droplet wetting transition caused by inhomogeneous microscopic structures can influence the evaporator liquid distribution,heat transfer,drainage and heat recovery processes.Therefore,analyzing the droplet wetting state and studying the surface microscopic characteristics,defects or disturbances influences on the surface droplet,can provide supportment for improving the wetting theory,understanding the formation mechanism of the droplet wetting state and developing hydrophobic heat transfer surface with high interface free energy.By using the methods of the formula derivation,numerical simulation and experiment research,the hydrophobic nonspontaneous transition regime was proposed and the problem of whether the trigger condition should be used in the wetting transition process was settled.The influences on the droplet wetting state and the gas-liquid interface stability by the structure size,morphology,hierarchy,defects and evaporation were aslo studied in this paper.The main work of this paper is as follows:By considering the change in interfacial free energy per unit droplet basal area,the surface droplet wetting states were divided,the state equations were presented and depicted in a cosine figure.The influence factors on the wetting state and wetting regime distribution were discussed.The results indicate that the surface droplet wetting states can be divided into five regimes and the hydrophobic surface can be furtherly divided into the spontaneous Cassie-Wenzel regime and the nonspontaneous Intermediate regime by the critical contact angle.The final wetting state of the droplet must be corresponding to the state equation closest to 0 axis.Increasing the roughness coefficient or solid-liquid contact area fraction is benefit for fabricating superhydrophobic surfaces.The surface microscopic characteristics,such as structure scale,size and morphology influences on droplet wetting state were studied.The influences on droplet wetting state stability by the interior structure angle were analyzed by derived droplet wetting state stability analyzing equation.The results indicate that the stability of the inverted trapezoid microstructure is the best,but adding depth will weaken the stability.Large interior structure angle is benefit for the droplet maintain Cassie-Baxter state,small interior structure angle is good to control the process and schedule of the wetting transition.In this part,the influences by the structure hierarchy and defects were simulated and analysed.The microstructure surfaces were produced by controlling surface modifying time.The wetting state stability of the evaporated droplet was studied by experiments.The results indicate that two-stage microscopic structure can reduce the droplet rolling charateristics.The stability of the meniscus is weakened by different depths of the surface microscopic characteristics.The droplet final wetting state is varing with the stage spacing ratio.The inner region morphology within the triple phase line has less influence on the droplet wetting state.The scaling is bad for surface drainage.Evaporation can induce the instability of meniscus and the morphological changing.The gas-liquid interface of the microgrooved surface is more stable than the interface of the micropillar surface.The microgrooved surface can prevent the droplet wetting transition from the Cassie-Baxter state to the Wenzel state more effective than the micropillar surface.