Dynamics Of Bubble Growth And Drop Impact On Complex Substrates

The surface of simple substrate has various shortcomings,such as single property,insufficient practicality,and narrow application range and so on.In recent years,the structured complex surface with special characteristics such as superwettability and low fluid flow resistance has caused researchers' attention.In nature,lotus leaves,butterfly wings,cicada wings,rose petals and mosquito eyes have various special properties due to their complex micro-nano structures or surface energy substances,which are important for the artificial manufacture of complex surface materials.In the past few decades,with the rapid development of science and technology,researchers have used various technologies to prepare various types of complex surfaces,such as superhydrophobic,superhydrophilic,superoleophilic,superoleophobic,microscale column/ hole array structured surface,etc.,and they are used in various fields such as self-cleaning,antiicing,anti-fogging,heat transfer,liquid-liquid separation,energy conversion devices,etc.For the obtained complex surface,it is often necessary to further characterize the phenomenon that occurs after the drop impacts and the dynamic process analysis of bubble growth on the complex surface.Therefore we designed three kinds of complex substrates in this paper,and studied the dynamics of bubble growth in drop and drop impact on complex substrates.The main research points of the full text are as follows:(1)The dynamics of bubble growth have been studied extensively for decades.However,there is still a lack of understanding of the morphological evolution of bubbles inside drops due to the coalescence of adjacent bubbles caused by the expansion of bubbles inside the drop on the substrate with cylindrical-patterned hole sturucture.Therefore,the first type of complex substrate we designed is cylindrical hole array substrate in this paper.Firstly,the relationship between the critical atmospheric pressure and the parameters of the cylindrical hole array is solved for the drops resting on the complex substrate surface with the cylindrical hole array based on the Laplace pressure balance at the gas-liquid interface,whether bubbles are generated by the trapped gas expansion in the microscale holes at the bottom of the drop.Secondly,the bubble growth and coalescence process were quantitatively analyzed by the change of the gas area ratio on the surface with the cylindrical hole array.And the differences of the bubble growth and coalescence process under the different atmospheric pressure difference of chambers were solved by using this quantitative method.Finally,the relationship between the degree of bubble coalescence and the ambient pressure difference and the parameters of the cylindrical cavity array on the substrate is analyzed.The Rayleigh Plesset model and the equation of state of ideal gas are used to quantify the relationship between the delay time and the final equilibrium bubble coalescence degree and the atmospheric pressure difference of the chamber and the parameters of the cylindrical cavity array on the substrate,respectively.These results provide insight into the underlying physical properties of bubble growth on a pore-patterned surface,which is important for the study of gas-liquid-solid interface slip and surface drag reduction.(2)Furthermore,the expansion of the bubble inside the drops on the substrate with cylindrical-patterned structure will drive the slip of the thriple-phase contact line outside the drops.The dynamic contact angle and contact diameter with time are obtained by analyzing the change of drop profile with time.The results of bottom view experiments are further demonstrated.In addition,the relationship between the increasing distance of the water drop thriple-phase contact line and the parameters of the cylindrical porepatterned structure is quantified,and the relationship between the critical contact angle and the cylindrical pore-patterned parameter is analyzed according to the contact angle theory on the substrate with cylindrical pore-patterned structure.The results of the study has potential application value in the optimization design of self-cleaning structured surface.(3)Dielectrowetting technology is an important method to control surface wettability.Therefore,the second type of complex substrate we designed is the dielectrowetting substrate in this paper.And the impact of a water drop on a surface with adjustable wettability is studied by combining the dielectrowetting technique with high-speed photography.First,the structure of the dielectrowetting substrate and the related theoretical basis are introduced,and the performance of the dielectrowetting substrate prepared in this study is quantified.Secondly,the phase diagram between the drop phenomena after impact,the wettability of the substrate surface and the Weber number of the impact drop is obtained,and the reasons for the occurrence of various drop phenomena are explained.Finally,parameters such as drop spreading factor,oscillation period,size and velocity of jet droplet are selected to quantitatively analyze various drop phenomena.The study of drop impact based on the dielectrowetting effect provides a new direction for the study of drop dynamics.In addition,the preparation method of the substrate with adjustable surface wettability can be applied to industrial fields such as inkjet printing and coating preparation.(4)The impact of water drops on solid substrates or liquid layers has been extensively studied,but there are few studies on the impact of water drops on uniformly distributed monolayer particles,especially the distribution of particles after impact.Therefore,the third type of complex substrate we designed is the hydrophilic/superhydrophobic substrate covered by a single layer of particles.First,the distribution of the particle layer after the millimeter-sized water drops hitting the uniformly distributed monolayer of polymethyl methacrylate(PMMA)particles on the hydrophilic/superhydrophobic glass slide substrate was studied.Secondly,analyze the mechanism of particle distribution formation after drop impact,specific composition and the relationship with the initial impact velocity of drop.Finally,the relationship between the difference in the maximum spreading diameter of the drops under the system and the wettability of the substrate and particle materials is analyzed.These findings contribute to the understanding of drop impact on complex surfaces,and have important practical application value for droplet impact related industries.