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

Abstract

The wetting and adhesion characteristics of 20 different surfaces have been studied systematically by both static water contact angle (θ) and dynamic contact angle measurement techniques: sliding angle (α) and advancing (θA) and receding (θR) 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 θ at ∼71° to superhydrophobic with θ exceeding 150°. Plots of θ of these surfaces versus α, (cos θR – cos θA), and the contact angle hysteresis (θA – θR) all yield scattered plots, indicating that there is little correlation between θ and α, (cos θR – cos θA) and (θA – θR). 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 θA decreases and becomes zero when θA is >150°. 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 α, (cos θR – cos θA) and (θA – θR), and found very little correlation. On the other hand, the pull-off force is found to correlate well to the receding contact angle θR. Specifically, pull-off force decreases monotonically as θR increases, suggesting that θR is a good measure of surface adhesion. Very interestingly, we also observe a qualitative correlation between θR and the quality of the pull-off. The pull-off was found to be clean, free of water residue after pull-off, when θR is >∼90° and vice versa. The implications of this work toward surface contact angle measurements and print surface design...

Meaningful Contact Angles in Flotation Systems: Critical Analysis and Recommendations

“…This hypothesis is well supported by recent studies on adhesion between water droplets and solid surfaces using a microbalance. 57 It was demonstrated for a number of polymeric surfaces that the pull-off force measured to separate a water droplet from a polymer surface related directly to the receding contact angles. Since the authors used a reversed system, this translates to advancing contact angles in separation of mineral from the bubble surface.…”

Physics and applications of superhydrophobic and superhydrophilic surfaces and coatings

“…The centrifugal adhesion balance, introduced in 2009, uses centrifugal and gravitational forces to induce different normal and lateral force combinations for direct adhesion measurements between a liquid drop and a solid surface. 29 Then in 2011, Samuel et al 60 introduced a microbalance with a specially designed hydrophobic loop to hold a liquid drop capable of measuring the liquid-solid surface interactions on drop-surface approach (snap-in force) and pull-off force (adhesion) during liquid drop detachment. The authors found for a large number of samples having various surface characteristics that advancing contact angles correlate better with the snap-in force whereas receding contact angle with the pull-off force.…”

Direct Measurements of Adhesion Forces for Water Droplets in Contact with Smooth and Patterned Polymers

“…For example, Liu et al 35 studied the interaction between a liquid droplet and a series of gold substrates with wettability gradient by using a microbalance, and they found that the adhesion force decreases as the contact angle increases. Samuel et al 36 studied the adhesion of water droplets with 20 different surfaces, from smooth and flat to rough and artificially textured, by using also microbalance. They showed that pull-off forces decrease monotonically with an increase in the receding contact angle and suggested that the receding contact angle is a good indicator of the liquid-solid adhesion.…”