Rose Petals and Lotus Leaves: Nanotechnology lessons from Nature

Many naturally occurring surfaces (feathers of a duck, feet of a lizard, lotus leaf) are super-hydrophobic, i.e., the contact angle for water is very high (>140?). However, some natural super-hydrophobic surfaces show an additional interesting property. Both rose petals and lotus leaves have super-hydrophobic surfaces, but while a water drop quickly rolls off a lotus leaf, it sticks to a rose petal so well that it does not fall off even on inverting the petal! Whether the hydrophobic surface is ‘slippery’ (lotus leaf) or ‘sticky’ (rose petal) cannot be characterized by just the contact angle, and requires one more parameter: the contact angle hysteresis. The coexistence of these apparently contradictory properties – strong adhesion and super-hydrophobicity – is usually ascribed to the complexity of the surface microstructure. In this talk, I will describe how the hierarchical nature of the surface – structured at both the micro and the nano scale – can give rise to a whole range of surface properties. Then, I will describe how the emergent behavior exhibited by a clustered metal nanorod array, can be utilized to produce a super-hydrophobic surface with a two-level hierarchical structure required for tuning the contact angle and the contact angle hysteresis. Suitable optimization of the cluster geometry in metal nanorod arrays can also lead to large enhancements in field-induced electron emission and laser-induced x-ray emission.