四棱锥复合梯度表面液滴的定向移动

Directional Droplet Movement Based on Spatial Quadric Pyramid Composite Surface

目的探究出能使液滴移动距离达到最大的复合表面制备手段,以及液滴在其表面的移动机理及影响因素。方法采用电火花加工技术在H62黄铜表面制备四棱锥结构构建结构梯度,并通过化学刻蚀在四棱锥表面制备润湿梯度,从而形成复合梯度结构。分别使用扫描电子显微镜和高速摄像机观察复合表面的微观结构与试件表面液滴的移动过程。结果复合结构表面不仅具有不对称几何形状引起的拉普拉斯压力,其上的液滴还会受到不平衡表面张力的作用。润湿梯度提供的不平衡表面张力与结构梯度形成的拉普拉斯压力共同组成驱动合力,令液滴在相较单一梯度表面表现出更强的移动性能。对比分析侧面夹角等因素对液滴定向移动的影响规律后得出,经化学蚀刻后的复合表面,液滴移动距离提升明显,并且移动距离增加量与夹角β成正比。在结构梯度与润湿梯度的共同作用下,液滴在β=5°的试件表面上定向移动距离达到最大,最大移动距离Lmax=6.96 mm。结论相较于单一梯度表面,复合型液滴定向移动表面可以获得最长的移动距离,表现出良好的应用前景。制备方法上,电火花线切割与化学刻蚀操作简单,成本低廉,有助于推动制备大批量液滴定向移动复合表面的相关研究。

Directional movement of liquids has a wide range of applications in fog collection,oil-water separation,microfluidics,condensation and heat exchange,protein adsorption,and cell migration.It is found that organisms such as cactus spines and spider silk have the ability to drive the directional movement of liquid droplets due to the presence of structural gradients on the surface.Inspired by this,a composite structure surface is prepared in this work.Wire electrical discharge machining(WEDM)was used to prepare a tetragonal structure on the surface of H62 brass,and wetting gradient was prepared on tetragonal surface by the stepwise chemical etching.500 mL NaOH solution with a concentration of 2.67 mol/L was mixed with 500 ml ammonium persulfate solution with a concentration of 0.15 mol/L.The cone tip of the pyramid structure was vertically fixed at the bottom of the container,and the computer program was used to control the power device to push the syringe every 20 min to inject the same amount of mixed solution to complete the construction of the wetting gradient.Thus,the unbalanced surface tension provided by the wetting gradient and the Laplace pressure formed by the structural gradient formed a driving force,making the droplets exhibit stronger motion performance than a single gradient surface.Scanning electron microscope and high-speed camera were used to observe the microstructure of the composite surface and the movement of droplets on the surface of the specimen,respectively.At the same time,the effect laws of the lateral angle and other factors on the droplet directional movement were compared and analyzed.Under the action of the structural gradient of the unetched surface,the driving force of the droplet decreased with the increase of the angleβ,so the droplet moving distance was negatively correlated with the angleβ.After a certain wetting gradient was given to the surface,at the same position from the tip,the side width of the specimen increased with the increase of the angleβ,so that the unbalanced tension of the wetting gradient on the droplet also increased.Therefore,compared with the unetched pyramid surface,the movement distance of the composite surface droplets increased significantly,and the increase of the droplet movement distance was positively correlated with the angleβ.Finally,under the combined action of structural gradient and wetting gradient,the directional movement distance of droplets on the surface of the specimen withβ=5°reached the maximum,and the maximum movement distance Lmax=6.96 mm.In conclusion,the composite surface preparation method that can maximize the droplet movement distance is explored,and the directional movement performance of droplets on the surface and its affecting factors are also studied.Compared with the single gradient surface,the composite droplet directional movement surface can obtain the longest movement distance.The preparation method of wire electrical discharge machining and chemical etching is simple and low cost,which is helpful to promote the preparation of large quantities of researches on directional movement of droplet based on composite surface,showing a good application prospect.