General Aspects of Biomimetic Materials - Biotechnologies and Biomimetics for Civil Engineering - page 67

instead of a film. Most of the times, superhydrophobicity is coupled with a low
tilting angle (i.e., the angle at which a surface has to be tilted in order for the drop
to slide), so that drops easily roll off from the surface. The strategies used to
achieve hydrophobicity span from hydrophobic waxes (e.g., grapes, Koch and
Ensikat 2008 ), air-retaining microstructures (e.g., Salvinia fern, Barthlott et al.
2010 ) and high hair density (e.g., Alchemilla monticola, Otten and Herminghaus
2004 ), to hierarchical topographies (e.g., lotus, Bhushan et al. 2009 ).
One of the best known examples of the impact of hierarchical topography on
hydrophobicity is given by the lotus (Nelumbo nucifera), a plant whose leaves
have one of the highest known contact angles (164). This very low wettability of
the lotus results from a combination of a regular pattern of micropapilae (convex
structures *20 lm) with superimposed wax crystals, (Bhushan and Jung 2011 ).
Other combinations of micro and nano-patterns with impact on surface wettability
have been observed in organisms like butterflies (Yan et al. 2007 ), english weeds
(Pereira et al. unpublished results, Fig. 3.4 ), rice leaves (Feng et al. 2002 ), and rose
petals (Feng et al. 2008 ).
Fig. 3.4 Scanning electron microscope image of epoxy replicas of a the underside of the English
weed leaf (scale bar = 200 lm), b the petals of the flowers of English weed (scale bar = 40 lm),
c the petals of a Cape Daisy (scale bar = 60 lm) and d the petals of a rose (scale bar = 40 lm).
(Pereira et al. unpublished)