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|Extreme Behavior Gets All the Attention|
If you have kids, you know that it's
the extreme behavior that gets all the attention. After all, the
school principal isn't going to call home to report "normal"
behavior. It's the extreme behavior that gets him on the phone.
The same seems to be true with contact angles. It's the far extremes, superhydrophobicity and superhydrophilicity, that seem to get all, or at least most of the attention.
It's generally accepted that superhydrophobicity occurs when the water contact angle exceeds 150°.1 We know that no smooth surface can naturally be superhydrophobic. In fact, the highest water contact angle that can be achieved on a smooth surface is in the neighborhood of 120°. In order to exceed that limit, a surface requires a nanostructure or a microstructure or both.2 We also know that when a surface is already hydrophobic (i.e., the water contact angle is greater than 90°), any increase in roughness will amplify the hydrophobicity resulting in a higher contact angle.3 Interestingly, roughness will make a hydrophilic surface even more so. Thus, you could say that roughness amplifies contact angle behavior pushing in the direction of the extremes - one way or another.
If superhydrophobicity is not extreme enough for you, then we then have ultrahydrophobicity. It's generally accepted that an ultrahydrophobic surface will exhibit a water contact angle in excess of 170°. Nature abounds with ultrahydrophobic surfaces. The lotus leaf is a famous example due to it's amazing hierarchical structure and self-cleaning capabilities. The term "lotus effect" got its name from the lotus leaf and is used to refer to an ultrahydrophobic surface that exhibits a hierarchical structure.
Other examples in nature include plants like the prickly pear and flowers of the genus Tropaelum (found in Central and South America). Some insects such as the water strider and the whirligig beetle have super- and ultrahydrophobic surfaces. Some birds (including doves and penguins) have superhydrophobic wings to keep the rain off and allow water to roll off before it can freeze.4
On the other end of the spectrum is the surface with extreme wetting properties which is said to be superhydrophilic and will exhibit a water contact angle close to 0°. This behavior is also sometimes called superwetting and is less commonly studied by surface scientists but still a subject of interest to some.
Plasma treatment is an example of a surface modification used on polymers to make a hydrophilic surface superhydrophilic. The surface is bombarded with electrons with a greater energy than is needed to break the molecular bonds which produces free radicals and increases surface energy. For this reason, plasma treatment is often used to improve adhesion.
As scientists gain a greater understanding of the extremes in contact angle - superhydrophobicity and superhydrophilicity - new technologies and methods will blossom to solve all types of world problems. The Namid Beetle comes to mind as the prototypical example in nature of using both hydrophilic surfaces (to collect water) and then hydrophobic troughs (to channel the water to the beetle's mouth). Cloud catchers which mimic this extreme behavior at both ends of the contact angle spectrum are being deployed in dry areas of the world to collect water for agricultural production and human consumption.5
However, some researchers want to up the bar by raising the minimum
contact angle which can be called superhydrophobic at 160°.
|Tools for the Extremes|
Whether you're measuring super high or
super low contact angles, we've got the instrument for you. All of
our models are adept at measuring super- and ultrahydrophobicity. We
have no problem measuring contact angles close to 180°.1
At the other extreme, however, contact angles below 5° can be challenging to capture. For this reason, we've developed a proprietary overhead method which relies on the drop volume and the diameter of the three-phase line in order to automatically calculate the contact angle in the software. For more information, including a PDF Brochure, video, and pictures, see http://www.ramehart.com/accessories.htm#100-31.
1 This video shows
contact angles in the low 170s being measured by a ramé-hart
goniometer on an ultrahydrophobic surface: