Superhydrophobic surfaces Dr Chiara Neto
| The leaves of some plants naturally possess the ability
to repel water droplets, and to remain completely dry after rainfall,
even though the material that coats their surface, epiticular wax, is
not particularly hydrophobic. It has now been clear for more than a
decade that such behaviour is not generated by surface chemistry alone,
but by a synergy between a particular double-scale (micro- and nano-)
surface roughness and surface energy. Surfaces, such as the leaves of
the lotus plant, on which water forms a very high contact angle (above
150 degrees) are called superhydrophobic, and are being employed in many
different applications. The particular combination of nano-scale and micro-scale roughness produces a self-cleaning behaviour: water droplets roll off (instead of sliding off) the surface of the leaf, and in doing so carry with them small dirt particles that soil the surface; this self-cleaning ability is connected to a very low value of contact angle hysteresis (the difference between the advancing and receding contact angle), and artificial analogues of the lotus leaves are being proposed as self-cleaning coatings that reduce the adhesion of snow and rain. Superhydrophobicity stabilises a thin layer of air on surfaces immersed in water, making superhydrophobic surfaces promising candidates as anti-fouling coatings for ships, and as means to transfer liquids without loss. But it is particularly in microscale and nanoscale devices that the true potential of superhydrophobic surfaces lies. Due to the large surface/volume ratio in microfluidic devices and MEMS, surface forces such as adhesion, friction, capillary forces, and viscous drag forces become dominating with respect to inertial forces, and it is therefore particularly important in these systems to develop lubrication coatings that will facilitate flow and movement of parts. Also see: Neto C, Joseph KR, Brant WR. "On the superhydrophobic properties of nickel nanocarpets." Physical Chemistry Chemical Physics. 11, 9537-9544 (2009). KR Joseph, C Neto,; “On the superhydrophobic properties of crystallised stearic acid” Austr. J. Chem. 63, 525-528 (2010). |
 These movies were captured during the third year workshop at the School of Chemistry of the University of Sydney. Click on the links below to see the movie in .wmv format Bouncing stream (862 kB) Bouncing droplet (2050kB) Self-cleaning (3100 kB)
Copyright The University of Sydney.
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