Mechanism Study On Shape Evolution And Wetting Transition Of Droplets During Evaporation On Textured Surfaces

Droplet evaporation is a normal nature phenomenon,and also has important background in our daily life and industry.There is wide application prospect in the area of biochemical detection etc,as superhydrophobic surfaces have the function of pre-enrichment during the evaporation process of a droplet containing solid particles.Consequently,the droplet evaporation process on superhydrophobic surfaces has attracted significant interest.So far the droplet evaporation on superhydrophobic surfaces has been experimentaly studied extensively.However,it is not clear about the mechanism on shape evolution and wetting transition of droplet during evaporation on superhydrophobic surfaces.In this study,the mathematical models and physical models were developed to describe the shape evolution and wetting transition during droplet evolution process on superhydrophobic surfaces through the analysis of the driving force and resistance force on the three phase contact line(TPCL)of an evaporation droplet based on the calculation of the gradient of interface free energy(IFE).The mechanism of droplet evaporation mode changing and wetting transition were quantitatively investigated.Firstly,the changing rules of the TPCL and contact angle were measured about droplet evaporation on hydrophilic,hydrophobic and superhydrophobic surfaces by experiments.Then,the mathematical model of the TPCL and contact angle changing of droplet during evaporation was developed to explain the above experimental phenomenon.Calculation results of the model show that when the driving force is smaller than the resistance the TPCL is pinned,and the contact angle decreases.In other words,the droplet evaporats in the constant contact line(CCL)model.However,when the driving force is larger than the resistance the TPCL starts to shrink while the constant contact angle(CCA)is constant.Then,the expressions of IFE of composite droplets with meniscal liquid-air interface in metastable state on micro/nano textured surfaces were formulated.The parameters to describe the meniscus beneath a metastable composite droplet were determined based on the principle of minimum IFE.Furthermore,the IFE barriers and the necessary and sufficient conditions of drop wetting transition from Cassie to Wenzel(C-W)were analyzed and the corresponding criteria were also formulated.The results show that the liquid-air interface below a composite droplet is flat when the post pitches of rough surfaces are relatively small,but may be in a shape of curved meniscus when the piteches are comparatively large and the curvature depends on structural parameters.The angle between meniscus and pillar wall is just equal to the supplementary angle of intrinsic contact angle of post material.The calculations also illustrate that Cassie droplets will transform to Wenzel state when post pitch is large enough or when drop volume is sufficiently small since textures are completely infiltrated and the energy barrier of transition plus wetting work is less than zero.The opposite transition from Wenzel to Cassie state,however,is unable to take place spontaneously because the energy barrier is always positive.Finally,the calculation results of this model are well consistent with the experimental observations in literatures for the wetting transition of droplets from Cassie to Wenzel state.Next,based on the analysis of energy and its gradient of a droplet,the mechanism of C-W transition was further analyzed.The interface free energy gradient of a small droplet was firstly proposed and derived as the driving force for its C-W evolution in this study based on the energy and gradient analysis.Then the physical and mathematical model of C-W transition was developed after the C-W driving force or transition pressure,the resistance,and the parameters of meniscus beneath a droplet were formulated.The results show that micro/nano structural parameters significantly affect the C-W driving force and resistance.The smaller the pillar diameter and pitch,the minor the C-W transition pressure and the larger the resistance.Consequently,the C-W transition is difficult to be completed.Meanwhile,a droplet on a textured surface with short pillars is easy to complete its C-W evolution.On the other hand,the smaller the droplet,the easier the C-W shift since the transition pressure becomes larger.Beside,both the intrinsic and the advancing contact angles affect C-W transition as well.The greater the two angles,the more difficult the C-W transition.Furthermore,in this study based on the droplet IFE always changing from high to low,the C-W transition process after meniscus touching substrate(MTS)mechanism was divided into different stages.Then,the changes of droplets IFE after MTS were analyzed.And the resistance on TPCL was also investigated.The criterion formula was derived so that the physical and mathematical models were developed.The calculation results show that the C-W transition of droplets is influenced by characteristic parameters of micro/nano structure surface.Finally,the comprehensive physical and mathematical model was developed to describe the shape evolution and wetting transition during the whole evaporation on superhydrophobic surfaces.And then,the mechanism of the whole process of droplet evaporation was interpreted by the quantitative calculation.The results show that a Cassie state droplet will evaporate in CCL model firstly.Then,it transits to CCA model when the volume is reduced to the critical volume.Subsequently,the droplet completes its C-W transition.In Wenzel state,the droplet experiences another period of CCL state.Finally,it enters into the mixed model(MM)stage until the evaporation finishes.