"Mother Nature is the greatest teacher to mankind," researcher Weihua (Marshall) Ming told nanotechweb.org. "The dual-size surface roughness on, for instance, the lotus leaf has proven to be very effective in generating superhydrophobicity. Meanwhile, a raspberry fruit naturally demonstrates dual-size morphology. We simply connected the characteristics of these two "products" - raspberry and lotus leaf - to prepare our superhydrophobic films."

The researchers believe their coatings are easier and cheaper to make and more robust than other artificial superhydrophobic systems. As a result, the coatings should suit many applications.

"We reported a simple approach to generating micro- and nano-structures in a controlled manner," said Ming. "The use of conventional materials and simple chemistry enables possible wide applications of this approach."

To make the coatings, Ming and colleagues prepared raspberry-like silica particles by attaching amine-functionalized silica particles with a diameter of 70 nm to the surface of 700 nm-diameter epoxy-functionalized silica particles. The team linked the particles together by inducing a reaction between the amine and epoxy groups, forming covalent bonds. Each larger particle attached to many smaller particles.

The researchers then bonded the "raspberries" to a partially cured cross-linked polymer film formed from an epoxy-amine system. Dangling amine bonds at the surface of the raspberries linked to epoxy groups in the polymer. Finally, the team coated the film with a layer of poly(dimethylsiloxane) (PDMS) to provide hydrophobicity.

The superhydrophobic films had an advancing contact angle for water of about 165° and a roll-off angle for a 10 μl droplet of about 3°. Without the presence of PDMS the advancing contact angle for water was only 22°, i.e. lower than the value for a smooth epoxy surface (66°). This confirmed that in order to be superhydrophobic, the coatings needed both a dual-size structure and the surface hydrophobicity provided by the PDMS layer. Meanwhile, the advancing contact angle for water on a smooth epoxy-based film with a surface modified with PDMS was 107°.

"Since we can easily control the size of raspberry-like particles, we would like to further investigate the relations between surface texture and wetting properties," said Ming. "We'll also explore the possibility of making superlipophobic [lipid-rejecting] films with this approach."

The researchers reported their work in Nano Letters.