Colder Products Company (CPC),
Keywords: wetting, super hydrophobic surfaces
Summary:Super repellent surfaces that combine structure and inherent hydrophobicity continue to attract much attention from the scientific community. A search of the term “super hydrophobicity” in Google Scholar produces more than 25,000 hits. If a water drop is deposited and allowed to advance across a structured surface that exhibits super hydrophobicity, the apparent contact angle between the water drop and solid surface will be quite large. Super hydrophobic surfaces have been arbitrarily classified as having an apparent advancing contact angle of greater than 150° and a roll-off angle less than 10°. Unfortunately, large contact angles are notoriously difficult to measure accurately, due to a variety of experimental challenges. Even though these pitfalls have been discussed frequently in the last few years, there is not widespread agreement on the validity of many of the reported contact angles from super hydrophobic surfaces. This discrepancy has persisted, in part, due to lack of experimental evidence of how water interacts with microscopic or nanoscopic features that cover these surfaces. In the past few years, investigators have used scanning electron microscopy or confocal microscopy to observe these local interactions. Notably, an experimental study on the underlying mechanisms of structured super hydrophobic surfaces was recently published by Schellenberger, Encinas, Vollmer and Butt, Phys. Rev. Lett. 2016, 116 (9), 096101(6). After depositing small drops of water, they inclined their surfaces to initiate movement. The contact between the water and structured surfaces was examined with confocal microscopy. They observed that drops were suspended atop the protruding features and movement of water was different at the advancing and receding edges. At the advancing edge, the water interface descended downward and draped itself across the features. At the receding edge, water jumped from one feature to the next. As Butt and coworkers did not test their data against any existing model, it is done in this paper. Here, a previously proposed model that employs linear averaging at the contact line was adapted to their surfaces in an attempt to estimate their contact and sliding angles. Predictions from the model generally agreed with their experimental measurements.