梯度润湿性ZnOHF薄膜上液滴定向铺展行为

Effect of Directional Spreading Behavior of Droplets on Gradient Wettability ZnOHF Films

目前对ZnOHF的润湿性相关的研究较少。借助ZnOHF的光响应润湿性转变特性制备梯度润湿性表面,研究该表面上的液滴定向铺展行为,验证应用该表面可提高潮湿环境中的集水量。通过水热法制备花状ZnOHF薄膜,基于其光响应特性对ZnOHF薄膜进行不同时间的紫外光照制备单侧梯度润湿性表面,研究液滴在梯度表面上的定向铺展行为。采用XRD和SEM表征产物的物相组成和微观结构,使用FT-IR表征紫外光照前后ZnOHF薄膜的-OH基团变化,使用接触角测量仪表征ZnOHF薄膜的润湿性,记录液滴的铺展行为。结果表明,在紫外光照射下,薄膜的接触角变化速度先快后慢,3 h即可由152°转化为0°。在梯度表面上水滴以7.42 mm/s的速度从光照起始线向超亲水端定向铺展4.08 mm,与均匀表面(1.16 mm/s)相比铺展速度更快,距离(3.82 mm)更远;梯度表面上的液滴可以克服重力铺展,其速度(3.84 mm/s)依然大于均匀表面。液滴在梯度表面定向铺展的主要驱动力为不平衡的表面张力,不同体积的液滴铺展到光照分界线处均出现加速现象。双侧梯度表面可以使小液滴汇聚于亲水区域,当液滴累积到一定体积时会向下渗透,达到集水的效果。模拟潮湿环境测试其集水性能,双侧梯度表面的集水量与原始表面相比提高31%。通过控制紫外光照制备梯度润湿性ZnOHF薄膜,研究液滴在梯度润湿性表面上的定向铺展行为,并验证了双侧梯度表面在集水方面的应用。

A surface with gradient wettability can realize the directional movement of droplets based on only its own characteristics without any external energy input,which makes it suitable for use in water collection,microfluidics,and self-cleaning.Therefore,flower-like ZnOHF films were prepared using a hydrothermal method and based on their photo-response characteristics,single-side wettability gradient surfaces were prepared using UV irradiation under different irradiation times.The directional spreading behavior of droplets on the gradient surface was then studied,and the phase composition of the product was characterized using an XRD analysis.The results show that the product of the hydrothermal reaction was ZnOHF with high purity and crystallinity,whose microstructure was observed using SEM.Based on the results,many flower-like ZnOHF were densely distributed on the surface of the mesh,which was composed of many slender nanosheets.Many of the nanosheet monomers converged at the same center on one end and diverged in all directions at the other end,indicating the structure is highly-branched.The morphology of the reaction process was observed and combined with an existing branching crystal growth mechanism,and the results showed that the flower-like ZnOHF was obtained via crystal separation.Moreover,the wettability of the ZnOHF films was characterized using contact angle measurement,and the surface changes on the ZnOHF film before and after UV irradiation were characterized using FT-IR.Before UV irradiation,the ZnOHF film was superhydrophobic,and the peak value of the-OH group was low,indicating that the number of-OH groups was less.After 3 h of UV irradiation,the peak value of the-OH group significantly increased,indicating that the number of-OH groups adsorbed on the surface of the ZnOHF film had increased,which shows the transition from superhydrophobic to superhydrophilic.However,the-OH group adsorbed on the oxygen vacancy was not stable,and the contact angle of the ZnOHF film with water was restored to 152°after 5 days of darkroom treatment.Additionally,the surface could still be converted to a superhydrophilic state after 3 h of UV light treatment.This process was repeated multiple times,and the results showed that the wettability conversion of the ZnOHF film is reversible.The gradient wettability surface was prepared by varying the UV irradiation time of the ZnOHF film,and the spreading process of the droplets was recorded using contact angle measurement.The results showed that under UV irradiation,the contact angle of the mesh first rapidly changes before decelerating,and it can be varied from 152°to 0°within 3 h.On the gradient surface,the water droplets were spread 4.08 mm from the light initiation line to superhydrophilic at a speed of 7.42 mm/s,which was faster(1.16 mm/s)and farther(3.82 mm)than that on the non-gradient surface.Moreover,the droplets on the gradient surface could also overcome gravity,and their velocity(3.84 mm/s)was higher than that on the non-gradient surface.The main driving force for the spreading of droplets on the gradient surface was the unbalanced surface tension;thus,the spreading of the droplets with different volumes to the light boundary was accelerated.Additionally,the two-side wettability gradient surface can also make small droplets converge in the hydrophilic region.When the droplets accumulated and reached a certain volume,they penetrated downwards to achieve the effect of water collection.In this regard,the water collection capacity of the two-sided gradient wettability surface was increased by 31%compared to that of the original surface.In summary,a gradient wettability ZnOHF film was prepared using controlled UV irradiation,the directional spreading behavior of droplets on the gradient wettability surface was studied,and the application of the two-sided gradient wettability surface in water collection was verified.