Kanungo Mandakini, Mettu Srinivas, Law Kock-Yee, Daniel Susan
Xerox Corporation Xerox Research Center, Webster 800 Phillips Rd, 147-59B, Webster, New York 14580, United States.
Langmuir. 2014 Jul 1;30(25):7358-68. doi: 10.1021/la404343n. Epub 2014 Jun 20.
Rough PDMS surfaces comprising 3 μm hemispherical bumps and cavities with pitches ranging from 4.5 to 96 μm have been fabricated by photolithographic and molding techniques. Their wetting and dewetting behavior with water was studied as model for print surfaces used in additive manufacturing and printed electronics. A smooth PDMS surface was studied as control. For a given pitch, both bumpy and cavity surfaces exhibit similar static contact angles, which increase as the roughness ratio increases. Notably, the observed water contact angles are shown to be consistently larger than the calculated Wenzel angles, attributable to the pinning of the water droplets into the metastable wetting states. Optical microscopy reveals that the contact lines on both the bumpy and cavity surfaces are distorted by the microtextures, pinning at the lead edges of the bumps and cavities. Vibration of the sessile droplets on the smooth, bumpy, and cavity PDMS surfaces results in the same contact angle, from 110°-124° to ∼91°. The results suggest that all three surfaces have the same stable wetting states after vibration and that water droplets pin in the smooth area of the rough PDMS surfaces. This conclusion is supported by visual inspection of the contact lines before and after vibration. The importance of pinning location rather than surface energy on the contact angle is discussed. The dewetting of the water droplet was studied by examining the receding motion of the contact line by evaporating the sessile droplets of a very dilute rhodamine dye solution on these surfaces. The results reveal that the contact line is dragged by the bumps as it recedes, whereas dragging is not visible on the smooth and the cavity surfaces. The drag created by the bumps toward the wetting and dewetting process is also visible in the velocity-dependent advancing and receding contact angle experiments.
通过光刻和成型技术制备了具有3μm半球形凸起和间距在4.5至96μm之间的凹腔的粗糙聚二甲基硅氧烷（PDMS）表面。研究了它们与水的润湿和去湿行为，作为增材制造和印刷电子中使用的打印表面的模型。研究了光滑的PDMS表面作为对照。对于给定的间距，凸起表面和凹腔表面都表现出相似的静态接触角，随着粗糙度比的增加而增大。值得注意的是，观察到的水接触角始终大于计算出的文泽尔角，这归因于水滴钉扎在亚稳态润湿状态。光学显微镜显示，凸起表面和凹腔表面上的接触线都因微观纹理而变形，钉扎在凸起和凹腔的前沿。在光滑、凸起和凹腔的PDMS表面上，静止液滴的振动导致相同的接触角，从110°-124°到~91°。结果表明，振动后所有三个表面都具有相同的稳定润湿状态，并且水滴钉扎在粗糙PDMS表面的光滑区域。这一结论得到了振动前后接触线目视检查的支持。讨论了钉扎位置而非表面能对接触角的重要性。通过在这些表面上蒸发非常稀的罗丹明染料溶液的静止液滴来检查接触线的后退运动，研究了水滴的去湿过程。结果表明，接触线在后退时被凸起拖动，而在光滑表面和凹腔表面上看不到拖动现象。在与速度相关的前进和后退接触角实验中，也可以看到凸起对润湿和去湿过程产生的拖动。
