The art of biomimicry has fascinated scientists ever since Leonardo da Vinci tried to fly like an artificial bird in the 15th century. Velcro, for example, was invented by mimicking the barb of a goose grass burr; the exquisite patterned shells of diatoms have provided the inspiration for sensors and nanovalves, and the way light reflects from butterfly wings is being explored for low energy displays.
Now, the amazing water-repellency of some plants is also receiving interest, particularly for its potential usefulness as selfcleaning materials. There are two different types of water-repellent plant leaves: hair-covered leaves, such as Lady’s Mantle, and macroscopically smooth leaves, such as Lotus. Water droplets run off both plants’ leaves and wash any dirt from their surfaces. Although the phenomenon is also observed on other plants’ leaves and animal feathers and hair, this ability has been termed the Lotus Effect. Materials exhibiting this effect are also called superhydrophobic materials. A superhydrophobic surface is defined as having a water contact angle greater than 150º and a very low roll-off angle. This unusual wetting behaviour has recently attracted considerable attention due to its potential applications for medical and industrial materials as well as for military textiles.
The idea of superhydrophobicity was fi rst discussed 60 years ago by A. B. D. Cassie, who worked for the British Council of the Wool Industries. In his article published in Nature in 1945, Cassie writes, ‘water always runs off a duck’s back because of the structure of feathers’ and ‘the mirror-like refl ection of raindrops on leaves, particularly noticeable with broccoli leaves, must be due to total refl ection at an air layer.’ Self-cleaning could therefore equally well be called the Duck effect or Broccoli effect!
Cassie and his colleague S. Baxter found that, on a suitable rough surface, the apparent water contact angle increases dramatically over the equilibrium contact angle, also known as the Young contact angle, θe, shown in Figure 1. They describe this effect by the following equation:
Cosθr = f1 Cosθe = f2
where θr is an apparent contact angle on a rough surface; f1 is the surface area of the liquid in contact with the solid divided by the projected area; and f2 is the surface area of the liquid in contact with air trapped in the pores of the rough surface divided by the projected area. If the top surface is flat, the above equation can be approximated by:
Cosθr = Φs Cosθe + Φs - 1= Φs (cosθe + 1) -1
where ΦS is the ratio of the rough surface area in contact with a liquid drop to the total surface covered by a liquid drop: this is the area of one blue square over the area of orange square in Figure 2; a is the width of a prominence of a rough surface; d is the distance between prominences; and h is the height of a prominence.
A smaller ΦS increases θr and makes the surface more hydrophobic. For example, for a water contact angle, θr, on a Tefl on (θe ≈ 120º) surface to be greater than 150º, the surface has to be roughened and the fraction of the surface in contact with water, ΦS, must be less than 26%. The remainder of the area under the droplet is occupied by air. Note that, for most fi bre applications, this approximation is not valid and the original Cassie- Baxter equation should be used.
If the structural features are of the appropriate geometry and magnitude, a surface can therefore become superhydrophobic and water droplets roll off easily when the surface is slightly tilted. Such an excellent water-repellent surface has high advancing and receding contact angles with very low contact angle hysteresis - the difference between the advancing and receding contact angles (Figure 3).
If similar behaviour could be obtained with low surface tension oil, droplets of these liquids would bead up on the surface and be easily removed. With reference to the defi nition of a superhydrophobic surface, we can defi ne superoleophicity as a surface having an oil contact angle greater than 150º and a very low roll-off angle. When an oil contact angle is greater than 150º, oil droplets easily run off a superoleophobic surface at a very low roll-off angle, washing dirt off in the process and effectively cleaning the surface. This has led to considerable interest by the military for applications in chemical and biological (CB) protective clothing and gear. Compared with hard materials, however, it is not easy to achieve superoleophobicity with soft materials such as textiles due to their lack of dimensional stability. Nevertheless, some scientists in academia and the military have never given up trying to develop such fabrics due to their potential importance in terms of protection against toxic liquids and vapours such as CB warfare agents.
Protecting the human body against toxic liquids and vapours is relatively straightforward using impermeable, gas tight suits and self-contained breathing apparatus. However, such equipment would be completely unsuitable for most military operations owing to its weight, limited wear time and high physiological burden. For this reason, CB military protective clothing is constructed from air-permeable materials in order to achieve an acceptable balance between protection and comfort.
Air-permeable CB protective clothing usually consists of two separate components: an outer fabric layer made from woven yarns, and an under layer containing an adsorbent material – typically activated carbon – to remove toxic vapour.
The outer fabric layer is treated with a fl uoropolymer oil- and water-repellent fi nishing agent, as in Figure 4, to prevent toxic liquids soaking into the textile layers and overwhelming the underlying adsorbent material.
These treatments contain comb-like polymers based on perfl uoroalkylated acrylates or polyurethanes and reduce the surface tension (γ) of individual fi bres to around 6-12 dynes/cm; γ for polytetrafl uoroethylene (PTFE) = 18 dynes/cm, for example.
The exceptionally low surface tension of these treatments means that the outer layer is able to repel not just water, which has a relatively high γ of 72.8 dynes/cm, but also liquid chemical warfare agents such as the blister agent sulphur mustard (HD) (γ ≈ 43 dynes/cm) and the nerve agents GD, GB and VX (γ ≈ 25, 24 and 30 dynes/ cm, respectively). Silicone or hydrocarbon waterrepellent treatments have much higher surface energies than fluoropolymer finishes and do not provide repellency towards these liquids.
Since the thickness of the fluoropolymer finish is extremely thin, in the order of microns, the air permeability of the fabric is unaffected and the clothing remains breathable, allowing the wearer to lose body heat by sweat evaporation.
In the case of a droplet of water, the combination of high contact angle and structural effects can result in very high apparent water contact angles on oil- and water-repellent textiles.
If the structural features of the fabric are of the correct geometry and magnitude, the result will be a superhydrophobic textile and water droplets will roll around freely on its surface. However, this is not the case for low surface tension liquids such as chemical warfare agents. Although oil- and water-repellent textiles effectively repel these liquids, they are not superoleophobic and so do not roll around unimpeded when the fabric is tilted, but rather become pinned to the surface.
In contrast to the phenomenon of superhydrophobicity, which has been studied in considerable depth, there have been very few studies of superoleophobicity. Whilst a recent paper has described the unsuccessful use of plant structures to achieve superoleophobicity, other workers have demonstrated that specific structural geometries can afford advancing and receding contact angles for hexadecane (γ = 27 dynes/cm) of 156° and 150°, respectively.
Clearly, the ability of a low surface tension liquid to roll effortlessly off contaminated clothing would greatly improve protection and aid decontamination procedures. Furthermore, such an effect would potentially eliminate contact hazards since the liquid agent would not adhere to the clothing in the first place. Superoleophobicity as well as superhydrophobicity is an area of research ripe for exploration, with many possible civilian and military applications in store in terms of protection against hazardous liquids and vapours.
Who knows, some day this cutting edge technology could help to make our everyday lives safer and easier.
Hoon Joo Lee is assistant professor of textile and technology management at North Carolina State University, US, and Colin Willis is a Dstl Fellow at the Defense Science and Technology Laboratory in Salisbury, UK.
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