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Langmuir, 2014, 30 (25), pp 7358–7368.
Mandakini Kanungo , Srinivas Mettu , Kock-Yee Law *.
Xerox Corporation Xerox Research Center, Webster 800 Phillips Rd, 147-59B, Webster, New York 14580, United States and
School of Chemical and Biomolecular Engineering,Cornell University, Ithaca, New York 14853, United States.
Abstract
Rough PDMS surfaces comprising 3 um hemispherical bumps and cavities with pitches ranging from 4.5 to 96 um 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.
Copyright © 2014 American Chemical Society.
Significant Statement
In addition to demonstrating the importance of rough geometry on wetting and de-wetting of rough surfaces, this work comprises several hidden points that are very fundamental to surface science. These points are often overlooked and they are:
- Liquid droplets are in metastable states when their static (apparent), advancing and receding contact angles are measured;
- The spreading of a sessile droplet on a solid surface is kinetic not thermodynamic control;
Contact angle is a 1D problem, not 2D. Specifically, contact angle depends on the energetic at the three phase contact line, not the area underneath the liquid drop.
