Designing hydrophobic surfaces for underwater applications
Lotus leaf is superhydrophobic and a water drops roll off easily. Since water does not stick on the leaf, if a boat is covered with lotus leaf on the outside will be encounter less water resistance and would be much easier to row. During the many centuries over which the water navigation technology was developed, it is fair to assume someone would have come up with this idea and even tried. So, why is that we don’t like the boats and ships with lotus leaf?
The reason is also based a common observation. If you have ever been served food on dried lotus lead you would have observed it casually. When exposed to water for longer time lotus leaf loses the superhydrophobic property and becomes wet. The figure above shows the small samples of the lotus leaf kept in water for 6 minutes at varying depths. The sample 1 on the left is kept at a depth of 30 cm while sample 5 is exposed to water at a depth of 5 cm. Clearly it is important for any underwater applications superhydrophobic surfaces have to retain its prtoperty for very long periods of time.
We started developing various surface textures and testing the surfaces water retention of hydrophobicity under water. Since superhydrophocity results from air trapped on the surfaces to ensure Cassie-Baxter state of inhomogenous wetting, we directly observed the trapped air. For this we used the concept of total internal reflection at the water-air interface. A schematic of the experimental setup used for measurement is shown in the figure. Light rays incident on the surface at the angles greater than the critical angle will undergo total internal reflection and would be captured by the objective of a long working distance microscope. Teh various surfaces we have used are shown below.
Mimicking Lotus
- Lotus has “Spikey” surface with more than one length scale. Mimicked with regular array of pillars on a silicon surface. Hydrophobicity is ensured with self assembled monolayer of FOTS.
- Lotus leaves becomes wet after 5 min of immersion! “Pillar” surface retains at least some air pockets up to 70 min.
- Super-hydrophobicity is reclaimed after drying
- Time lapse video
Mimicking Duck
- Duck feathers have stripes at different scales. Mimicked with thin ridges on silicon surface with FOTS coating.
- Air is trapped in between the ridges in the form of tiny bubbles.
- Air is retained for about 35 min.
- Time lapse video
Surfaces with tiny blind holes
- Air bubbles are trapped on the holes.
- Bubbles are independent of each other
- Time lapse video
- May be best for drag reduction