Wetting of Rough Surfaces by a Low Surface Tension Liquid

Abstract

There exist textured surfaces that demonstrate large advancing contact angles when the expected behavior is complete wetting due to high Wenzel roughness. The roughness can represent an impediment to the motion of the contact line, leading to the possibility of contact line pinning and thus increased advancing contact angle. A set of fabricated textured surfaces with varying pillar diameters and pillar spacing were tested using hexadecane. Because of the low surface tension of hexadecane, the majority of the surfaces exhibited penetration of the liquid into the roughness, also known as the Wenzel state. Because of this penetration, the empirical pinning force framework we previously developed for wetting behavior on smooth surfaces and surfaces with texture where liquid does not penetrate the roughness was expanded to include the Wenzel state. For the surfaces that had Wenzel wetting, the receding contact angle tended to follow the predictions of the Wenzel equation, while the advancing contact angle tended to increase with increasing roughness when according to the Wenzel equation it would be expected to decrease. For surfaces where nonpenetrated Cassie wetting was observed, a constant high advancing contact angle and a receding contact angle that follows the trend predicted by the Cassie equation were observed.

Thermodynamics of Surface Nanobubbles

“…This framework is very useful since it gives theoretical validity to the experimentally confirmed practice of cosine averaging of measured advancing and receding contact angles to estimate the Young contact angle for smooth surfaces and explains why cosine averaging should never be used for rough surfaces. 63 As well, the framework is able to provide an explanation for a wide variety of seemingly inexplicable experimental results for contact angles on rough hydrophobic, 63 wetting, 65 and oleophobic 64 surfaces. The framework also allows one to gain additional insight into contact angle phenomena by comparing the magnitude of the extra surface forces between different experimental circumstances.…”

“…There is independent support for both the contact angle framework of Koch, Amirfazli and Elliott,[63][64][65] and the gas enrichment layer. However, the validity of this section does not depend on the experiments for the gas enrichment layer, as we have not used any experimental values from gas enrichment layer supporting experiments.…”

Slippery Wenzel State

“…37 Therefore, validating these theoretical models would require ∆θ * to be much smaller than θ * . While earlier work has attempted to experimentally validate the classical Wenzel equation, [41][42][43] these experiments either report measured ∆θ * that are on the same order of magnitude as θ * , or do not indicate the magnitudes of ∆θ * that determine the possible experimental deviations from their measured θ * . On the slippery rough surfaces, all the measured ∆θ * are significantly lower than θ * (e.g., ∆θ * for water droplets are 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 13 hemi-wicking occurs.…”

Topographical and Physicochemical Contrast in Photopatterned SU-8 Films for Microfabrication of Multilayer Structures

“…Microposit resist displays better wettability compared to water. Wetting angles for water can be compared with typical values reported in the literature (60 ∘ -80 ∘ ) [31][32][33][34][35]. Water wettability is also important because the majority of developers products for positive tone resist are aqueous based solutions.…”