Lotus Leaves, Rose Petals and Nano-Soccer Balls: What the Fakir could these have in common? (No, I’m not using a bad word, check out the title of the respectable PNAS paper below 🙂
The Lotus Effect: A symbol of purity – the leaves of a lotus are always clean and dry. Water simply rolls off the surface, carrying dirt with it. This self-cleaning property is due to the superhydrophobic surface of the lotus leaf. Studded with micropapillae (image), water beads lie on the top like a fakir on a bed of nails. While ordinary waxy surfaces may have contact angles of >90° with the drop, fakir drops show extreme angles of 170°, minimizing contact to only 0.6% of their surface! In the virtual absence of adhesive forces, the drops skitter off, cleaning the leaf as they roll (The Lotus-Effect: CGI simulation).
The Petal Effect: While a water droplet on a rose petal is spherical, it does not roll off, even if the flower is held upside down. This is because the petal’s nanostructure allows the water to wet the space between the surface grooves, increasing adhesive forces (Wenzel model pix; http://goo.gl/GT7KF). Contrast this to the lotus leaf, where the drop sits on the ‘bed of nails’ with trapped air in between (Cassie-Baxter model pix; http://goo.gl/35Cz6).
Why do we care?: There is a wealth of nanotechnology drawing upon Nature’s brilliance. Watch this wonderful video: http://goo.gl/dXMQ2
More recently, scientists in the Netherlands figured out a way of packaging nanoparticles, that could be used to deliver drugs for example, into soccer ball-like clusters of very high density, by slowly evaporating solutions from a superhydrophobic effect. They show that fakir drops remain in the Cassie-Baxter mode without collapsing into the Wenzel mode. (Aren’t you glad you now know what that means!).
REF: Building microscopic soccer balls with evaporating colloidal fakir drops: