Effect of roughness geometry on wetting and dewetting of rough PDMS surfaces

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.