ramé-hart Newsletter

                  

December 2019
 

1. Perhaps the most famous natural surface for surface scientists is the Lotus leaf. Due to a hierarchal surface structure made up of both microscopic and nanoscopic topologies, the lotus leaf exhibits ultrahydrophobic self-cleaning properties. This surface is the gold standard for ultrahydrophobicity. Anyone who can create a manmade surface that is comparable with the lotus leaf would be considered a genius. So far, some have come close, but no one can best the lotus leaf. Commercial products such as Lotusan paint are based on the Lotus Effect.¹

2. The lotus effect relies on what is referred to as the Cassie state - that is, the drops are sitting on top of the asperities with air pockets underneath. A similar low-wetting behavior can be seen on rose petals. However, in this case, the drops are in a Cassie impregnated state. That is the voids under the drop fill in with water. While both the lotus effect and the petal effect exhibit equally high contact angles in excess of 150°, the lotus effect also exhibits extremely low contact angle hysteresis which results in a low roll-off angle. The same drop on a rose petal, by contrast, will exhibit high contact angle hysteresis and no roll-off angle. The lotus effect is often considered genuine superhydrophobicity while the petal effect is referred to as pseudosuperhydrophobicity.²

3. Geckos can defy gravity by walking on walls and ceilings. The Van der Waals force can explain the adhesive quality of gecko feet. The ubiquitous Post-it note product attempts to mimic this behavior.³

4. The water strider is able to walk on water with the help of its hydrophobic legs. In addition, thousands of hydrofuge hairs on its body keep it from staying underwater.⁴

5. The Namid desert beetle features both hydrophobic and hydrophilic surfaces. This allows it to collect water from the air and the roll it down its back and into its little mouth. Cloud-catching nets used in the Atacama desert copy the design of the Namid beetle to harvest water in one of the driest deserts in the world.⁵

6. Moth eyes have, in addition to anti-reflective properties, non-wetting properties. Researchers are attempting to mimic the the structure in order to develop more efficient solar panels and other commercial products.⁶

Astute surface scientists are always looking for models of good structure which can be emulated for specific applications. Inevitably the best of these models are found in nature and not surprisingly, the bioinspired manmade structures are a grade lower than the archtypes.

¹ See our April 2008 Newsletter.
² For a more detailed discussion of genuine superhydrophobicity and pseudosuperhydrophobicity, see our October 2015 Newsletter.
³ For a lively discussion of the Post-it Note, see our May 2012 Newsletter.
⁴ For more details on the water strider and other amazingly hydrophobic insects, see December 2015 Newsetter.
⁵ For more on these cloud catchers, see our February 2018 Newsletter.
⁶ For an expanded discussion of moth eyes, see our October 2009 Newsletter.
 

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