Study of Wetting and Adhesion Interactions between Water and Various Polymer and Superhydrophobic Surfaces

The wetting and adhesion characteristics of 20 different surfaces have been studied systematically by both static water contact angle (theta) and dynamic contact angle measurement techniques: sliding angle (alpha) and advancing (theta(A)) and receding (theta(R)) contact angles. These surfaces cover surfaces of all traits, from smooth and flat to rough and artificially textured. Fourteen of the surfaces are flat, and they range from molded plastic sheets to solution coated polymer films to chemical vapor deposition polymerized polymer films and to self-assembled monolayers on Si wafers. The rest of the surfaces include 4 fluorosilane coated textured Si wafer surfaces and two natural surfaces derived from the front and back side of the rose petal. Static water contact angle data suggest that these surfaces vary from hydrophilic with theta at similar to 71 degrees to superhydrophobic with theta exceeding 150 degrees. Plots of theta of these surfaces versus alpha, (cos theta(R) - cos theta(A)), and the contact angle hysteresis (theta(A) - theta(R)) all yield scattered plots, indicating that there is little correlation between theta and alpha, (cos theta(R) - cos theta(A)) and (theta(A) - theta(R)). Since the later three parameters have been mentioned to relate to adhesion semiempirically between a liquid droplet and the contacting surface, the present work demonstrates with generality that contact angle indeed does not relate to adhesion. This is consistent with a known but not well recognized fact in the literature. In this work, we study both the wetting and adhesion forces between water and these 20 surfaces on a microelectromechanical balance (tensiometer). When the water drop first touches the surface, the attractive force during this wetting step was measured as the "snap-in" force. The adhesion force between the water drop and the surface was measured as the "pull-off" force when the water drop separates (retracts) from the surface. The snap-in force is shown to decrease monotonously as theta(A) decreases and becomes zero when theta(A) is >150 degrees. The very good correlation is not unexpected due to the similarity between the wetting and the "snap-in" process. The analysis of the pull-off force data is slightly more complicated, and we found that the quality of the water-surface separation depends on the surface "adhesion". For surfaces that show strong adhesion with water, there is always a small drop of water left behind after the water droplet is pulled off from the surface. Despite this complication, we plot the pull-off force versus alpha, (cos theta(R) - cos theta(A)) and (theta(A) - theta(R)), and found very little correlation. On the other hand, the pull-off force is found to correlate well to the receding contact angle theta(R). Specifically, pull-off force decreases monotonically as theta(R) increases, suggesting that theta(R) is a good measure of surface adhesion. Very interestingly, we also observe a qualitative correlation between theta(R) and the quality of the pull-off. The pull-off was found to be clean, free of water residue after pull-off, when theta(R) is >similar to 90 and vice versa. The implications of this work toward surface contact angle measurements and print surface design are discussed.