By: Gert-willem Römer, Daniel Arnaldo del Cerro, R.C.J. Sipkema, M.N.W. Groenendijk, A.J. Huis in ‘t Veld

In nature the morphology of surfaces is used to tune material properties to the highest possible level. Self cleaning surfaces for example, like that of the lotus leaf, amplify the hydrophobic properties of wax crystals by superimposing them on a rough, microstructured surface. A lotus leaf remains always clean thanks to this effect. Figure 1 shows the effect of this on a water droplet situated on a lotus leaf. The goal of this research is to apply this structure to the surface of a number of materials by ultrafast pulsed laser ablation.

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Fig 1: The water repelling ability of the lotus leaf is a result of a microstructured surface covered with hydrophobic waxes.

Laser surface texturing tech¬niques have been successfully developed on micro-meter and nano-meter scale which makes surfaces super hydrophobic. This technique is being extended to surface textures which exhibit ice-phobic properties.

By laser ablation with ultra short laser pulses in the pico- and femto-second range, well controlled superimposed micro- and nano-scaled surface textures can be obtained. The microscale of the texture is mainly determined by the dimensions of the laser spot, whereas the superimposed nano-structure is the result of so-called laser induced “self organizing nanostructuring”. By controlling this micro-nano surface texture, it is possible to modify the natural hydrophobicity of materials, see figure 2.

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Figure 2: Two water drops on stainless steel. The left drop sits on top of a micro structured surface exhibiting a high contact angle; the surface on the right is smooth and wets the surface.

Anti-ice properties of these hydrophobic micro-nano surface textures are under investigation. It is believed that when applying such textures to the leading edges, engine inlets, etc. of airplanes, the poor wetting will prevent ice-accretion under certain icing conditions. Ice can distort the flow of air over the wing, reducing the aircrafts aerodynamic performance and leading to dangerous situations. Moreover, take-off is not permitted if ice contamination on the aircraft’s surfaces exists.

Popular materials in aviation have been laser-machined, and the resulting surface textures have been analyzed using optical and SEM microscopy. Coatings, on top of the textures, have also been applied to create superhydrophobicity. The hydrophobicity of the surfaces has been quantified by contact angle measurements, and the anti-ice properties of the surfaces have been tested in a climate chamber.

Wetting and ice accretion analyses will be performed, taking into account the fluid dynamics and heat transfer on macro and micro level. These results will be used to design and optimize an ice-phobic surface texture, considering the manufacturing process constraints and the durability requirements.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

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