Preparation Of The Nanostructured Superhydrophobic ZnO And La₂O₃ Surface And The Electromagnetic Wetting Performance

Extreme wetting state with contact angle?CA?greater than 150°,superhydrophobic solid surface has shown great potential application value in high-tech fields and daily life such as corrosion resistance,antifog,antibacterial,antifreeze,self-cleaning,oil-water separation,liquid lens and microfluidic control.With the continuous expansion of the application environment,its high stability in various extreme environments has become an inevitable requirement.However,the wetting state of many solid surfaces is very sensitive to the excitation of the external environment,such as pH value,temperature,external electric field,magnetic field,etc.This obviously affects the stability of the superhydrophobic surface,but on the other hand,people also get a great inspiration from it,and gradually developed more rich responsive wetting research,such as the electrowetting-on-dielectric?EWOD?has drawn great attentions due to its intellectualization,fast response,easy controllability,good repeatability,and so on.However,there still are some bottleneck problems in its further application,such as the fuzzy physical mechanism of electrowetting,obvious contact angle hysteresis,large saturated contact angle,and difficulty in achieving spontaneous reversible dielectric wetting.It is well known that wettability of solid surface mainly depends on the solid surface micro-nano structure and surface chemical composition.Therefore,people are still seeking new materials and preparing new surface microstructures to continuously improve the wetting behavior of solid surfaces,enhance the stability of superhydrophobic surfaces,and achieve large-angle reversible electrowetting.To achieve reversible electrowetting on the surface of structured solid,the dehumidification process of droplets needs to overcome a certain potential barrier.On the one hand,the potential barrier is reduced by surface modification to reduce the barrier to the liquid;on the other hand,additional energy is provided from the outside to overcome the barrier.As a far-field non-contact operation,magnetic field regulation forms electromagnetic wetting together with electrowetting,which is obviously an ideal way to provide additional energy to overcome the dehumidification barrier.Based on these considerations,we take the topic"Preparation of the nanostructured superhydrophobic ZnO and La₂O₃ surface and the electromagnetic wetting performance",and work from several aspects,and obtained a series of valuable results:?1?The ZnO nanocone array surface is designed and directly grown on the Zn substrate by using simple hydrothermal method to ensure a strong binding energy with substrate.Then every nanocone is coated with stearic acid?STA?from head to toe to be endowed the selfreparability,and further greatly improve its abrasion resistance and long-term durability.It attains superhydrophobicity with water contact angle of 162°.And it also exhibits superlow adhesion to water,perfect abrasion resistance,good water shock resistance,well thermal stability in the working environment temperature range below 200?for the ZnO based device.More importantly,it realizes the remarkable multiple protections to ZnO based devices in the various extreme environments,such as the high self-cleaning ability in the environments containing various solid and liquid pollutants,the well anti-corrosion performance in the acidic and basic environments,and the excellent freezing resistance in the harsh freezing conditions.?2?Herein,a more direct and simple one-step method including sol preparation process and dip coating is developed to fabricate the superhydrophobic Zn?OH?₂@stearic acid nanosheet coating,and Zn?OH?₂ is a kind of eco-friendly and non-toxic inorganic compound.The stearic acid?STA?,a typical low surface energy material,is directly precursor solution to form the sol,and then the closely packed Zn?OH?₂@stearic acid nanosheet coating is formed on substrate after the solvent evaporation.The sol is suitable various rigid and flexible substrates to obtain the superhydrophobic surface and the formed coatings exhibit the excellent mechanical stability between the coating and substrate.In various extreme environments,such as extremely cold,muddy,acidic,alkaline,high temperature,and long-term light irradiation,etc,the superhydrophobic coating has a good durability.In the oil-water separation,the superhydrophobic Zn?OH?₂@stearic acid nanosheet coating on flexible fabric has high separation efficiency about 99%and high flux about 1515 L/?m²?h?,and it is suitable for oil-water separation with high separation efficiency and flux in various extreme environments.?3?In these works,a ZnO nanocone array with superhydrophobic surface was prepared by using the hydrothermal method.On the one hand,the tip effect was first considered to enhance the horizontal force on the water droplet standing on the nanocone array surface through the enhancement of the local electric field near the cone tip under the applied voltage,and this further increased the spreading ability of the water droplet on the surface.On the other hand,the Cassie–Wenzel transition induced by voltage was suppressed by modulating the nanocone density and tip curvature,which correspondingly adjusted the surface tension of the water droplet between the two nanocones.Thus,the EWOD responses were measured and it was found that the EWOD response was divided into a low voltage response I and high voltage response II.The Cassie–Wenzel transition was observed only in the high voltage EWOD response II.Furthermore,the electric field distribution in the nanocone array was simulated by using the finite element method,and the EWOD response was further optimized by modulating the tip curvature of the nanocone.Obviously,this work was very beneficial to the achievement of spontaneous switchable wettability with a large CA modulation on a structured surface.?4?In the works,the La₂O₃ film has been prepared by using the simple hydrothermal method,and its structure is accurately controlled from the sparsely dispersed compacted microsphere to closely packed microsphere film when the hydrothermal temperature is below 180?.When the hydrothermal temperature is increased from 180?to 200?,the hierarchical structure including the sparsely dispersed compacted and flower-like microspheres,the discrete standing nanopetals,and the hierarchical structure constituted by the closely packed microsphere and densely standing nanopetals are formed successively.Eventually,the formed hierarchical structure including the closely packed microsphere and densely standing nanopetals are the optimized structure to enhance the EWOD responses.?5?A simple one-step hydrothermal method was successfully used to prepare a magnetic ZnO@STA nanorod array structure with superhydrophobicity and low adhesion on a Zn substrate,which is a vertically uniform micro-nano hierarchical structure.Based on the Hall effect,reversible electrowetting and electromagnetic driving are finally realized on the surface of the magnetic nanocone array,and the reversible adjustment range of the contact angle can reach 30°.