Bioinspired Self-cleaning Materials - Biotechnologies and Biomimetics for Civil Engineering - page 221

Fig. 9.12 a A designed rough surface (Reprinted with permission from Blossey 2003 ); b SEM
images of nanopillar array manufactured on quartz by mask-free lithography with PMMA resist,
inset contact angle (Adapted with permission from Kim et al. 2012a ); c SEM picture of a micro-
patterned polymer (KratonTM D-1102CS) film with gap size 5 lm and pillar height 10 lm
(Reprinted with permission from Peters et al. 2009 )
composite systems; superhydrophobicity was also achieved by controlled etching
of metallic surfaces, such as steel, copper, and aluminum (Bhushan et al. 2010 ;
Guo et al. 2011 ).
Here we list some outstanding examples of biomimetic self-cleaning surfaces,
and of how they were created.
In a work by Kwon et al. ( 2009 ) silicon surfaces were uniformly modified by
combining two etching techniques, generating a superhydrophobic nonclosely
packed roughness with contact angles higher than 170, low contact angle hys-
teresis and tilting angles lower than 2. The authors observed multiple bouncing of
water droplets on the surface, proving the onset of a Cassie nonwetting state.
A large-area substrate was converted by Lee et al. ( 2010 ) into a superhydro-
phobic surface by combining PMMA microspheres and a silicon grease, which
was turned into a nanostructured ceramic through electron irradiation: the obtained
microporous structure with ceramic nanobumps showed good mechanical stability
to standard tests.
In a recent work by Zhang et al. ( 2013b ) fluoro-silicone nanofilaments were
combined with Krytox liquids to produce a SLIPS structure and achieve antiw-
etting coatings inspired to Nepenthes pitcher plants (Fig. 9.13 ). Contact angle
increased with surface tension of liquids investigated, and a sliding angle of 4 was
recorded with both polar (water) and nonpolar (n-hexadecane, n-decane) liquids.
Groten and Rühe ( 2013 ) investigated more in-depth the effect of multiscale
hierarchical structures on surface wetting, by producing nanoscale, microscale or
combined micro and nanoscale roughness on silicon substrates by etching, and
coating them with a fluoropolymer. Nanorough surfaces showed contact angles
close to 180 and almost no contact angle hysteresis, but at the same time poor
mechanical stability was observed, while multiscale hierarchical structures with
identical chemical composition allowed both to reach superhydrophobicity and to
confer wear resistance to the surface.
Nevertheless, the latest advances in the field of self-cleaning surfaces were
proposed by Bird et al. ( 2013 ) who designed a fluorosilane-coated, laser-ablated