Biomimetics: clingy as an octopus or slick as a lotus leaf? Teach article

Astrid Wonisch, Margit Delefant and Marlene Rau present two activities developed by the Austrian project ‘Naturwissenschaft und Technik zum Angreifen’ to investigate how technology is inspired by nature.

In 2004, Alice Pietsch from the University of Teacher Education Styria^w1, Austria, was inspired by a simple but very visual demonstration at a science museum. A retired science teacher was using bellows to pump air into a pair of sheep’s lungs, rhythmically inflating and deflating them. The crowd around him was much larger than around many of the more high-tech exhibits – which is what prompted Alice to create an interactive science museum.

In 2008, her dream came true. Over five months, Styrian students of all ages developed 50 different activities for other students. The exhibition Naturwissenschaft und Technik zum Angreifen (‘Science and technology to touch’) took place in 2009 at the Haus der Wissenschaft (‘House of science’) in Graz, Austria, where the students and their teachers helped the visitors with each activity. It was a huge success.

Most of the activities from the exhibition are suitable for the science classroom; here we present two activities about biomimetics – the application of principles from nature to engineering and technology. Velcro, which mimicks the hooked seeds of burdock, and boat hulls that mimic the thick skin of dolphins are common examples. The activities below investigate suction pads and superhydrophobicity, both seen in nature. The activities were developed for younger secondary-school students (aged 10-15), but can easily be adapted for students of virtually any age and are a good opportunity for integrating physics, chemistry and biology. Depending on the level of detail you go into, the activities can each take anything from five minutes to over an hour.

The full collection of activities from the exhibition is available in print in German from Birgit Muhr (birgit.muhr@phst.at) for €19 plus postage.

Adhesion on flat surfaces: negative pressure

Household suction pads were inspired by the soles of tree frogs’ feet and the tentacles of octopuses, whose suction power was already known to the ancient Greeks.

We use these pads on flat surfaces – to attach hooks to bathroom tiles or make kitchen machines more stable, and on rubber mats in the shower or toy arrows. But why do they stick?

If you look at a suction pad closely, you will notice that it is slightly curved. Could this curvature be important for the pad’s adhesive power? And why do you need to wet suction pads before using them? Let’s investigate.

Materials

• 4 suction pads (make sure you can attach a string to them)
• A small fretsaw

• A 50 cm pole, about 3-4 cm in diameter (the students should be able to wrap their fingers around it easily)
• Four 50 cm pieces of strong string
• A pair of scissors
• A plastic board (about 1 x 1 m)
• A permanent marker pen

Procedure

1. Using the fretsaw, make four equidistant grooves in the pole.
2. Tie a piece of string to each of the four suction pads.
3. Tie the other ends of the string to the four grooves on the pole. Make sure the four lengths of string are exactly the same length.

4. 

   Put the plastic board on the ground, and use the permanent marker pen to draw four circles for the suction pads in a straight line. On either side of the line, draw a footprint (see image on right).

5. Press the suction pads onto the four circles.
6. What happens to the curvature of the suction pads when they are stuck to the plastic board?
7. Stand on the footprints and try to pull the suction pads off the plastic by pulling on the pole.
8. Repeat the experiment, wetting the suction pads before sticking them to the plastic board. What do you observe?

Discussion

When you press the suction pad onto the plastic board, you reduce the pad’s curvature, thereby reducing the volume of the space between the pad and the board, and causing some of the air between the pad and the board to escape. When you stop applying pressure, the elastic pad tends to resume its original, curved shape. The volume in the cavity between pad and board is increased again, but there is less air in it, and therefore also a lower air pressure. The higher pressure of the air outside the pad is what keeps the pad stuck to the board.

You can calculate the force of a suction pad as:

F = AP, where F: force; A: area; P: pressure.

The area will be πr² where r is the radius of the pad. The pressure inside the cavity between pad and board is negligible when compared to atmospheric pressure, which is about 100 000 Pa. So:

F = πr² (100 000 Pa)

The time it takes for the pad to come undone by itself (without applying additional force) will depend on how porous and flat both the rim of the pad and the underlying surface are, which determines how fast the air leaks back in and equilibrates the pressure.

Water, saliva and other liquids work well to seal the pad, making it more airtight and ensuring that air leaks back in less easily. Therefore, you need even more strength to pull the suction pads off if you moisten them before applying them to a surface.

Self-cleaning effects: hydrophobicity in nature

Nature has some real cleanliness fanatics: dung beetles crawl out of cowpats looking spotless, you rarely find a dirty butterfly or dragonfly, and some plants are very successful at shedding filth. The Indian lotus, for example, grows in muddy waters, yet no dirt clings to its leaves; in fact, in Buddhism, the lotus is a symbol of purity. You can find this lotus effect closer to home, too: nasturtium leaves and flowers look clean enough to eat, even without washing (rinse them anyway, just in case). How do they manage without soap?

Humans have replicated this self-cleaning effect for many purposes, for example, in self-cleaning plastics, roof tiles, windowpanes, ceramics, wood and car varnishes, and façade paint. You can also create dirt-repellent clothes by impregnating them with a special spray.

How does it work? Let’s find out.

Materials

• Parts of different plants with self-cleaning properties, such as nasturtium leaves (Tropaelum majus), lotus leaves (Nelumbo nucifera), cabbage leaves (Brassica oleracea), and tulip leaves (Tulipa spp.)
• Parts of different plants with hydrophilic leaves, such as leaves of the common beech (Fagus sylvatica) or the Southern magnolia (Magnolia grandiflora)
• Water, runny honey and / or liquid glue
• A Pasteur pipette and bulb, and some small spoons
• Ketchup, ground cinnamon and curry powder
• Newspaper, kitchen roll and textiles
• Impregnation spray (e.g. for waterproofing suede), self-cleaning nano-based façade paint, nano-coating for wood or glass
• Optional: a nano-sealed roof tile

Procedure

1. Wet the self-cleaning plants with water, honey or glue and cover them with ketchup, cinnamon or curry powder (to simulate dirt). What do you observe?
2. Repeat the experiment with the hydrophilic leaves. What do you observe?
3. Now investigate the self-cleaning properties of artificial materials. Spray textiles, kitchen roll and newspaper with different types of nano-sealing treatments (e.g. impregnating spray, paint or nano-coating). Either compare the effect of the same type of treatment on different materials, or of different types of treatment on the same material.
4. Use these and other nano-coated materials (e.g. tiles) to repeat the experiments, wetting and dirtying them. What do you observe?

Discussion

If you drop water or honey onto a lotus leaf, it will roll off very quickly. A close look under a scanning electron microscope reveals why: large numbers of tiny wax-covered bumps on the surface. These bumps are about 10-20 µm high and 10-15 µm apart from one another.

How does this structure help the lotus leaf to remain clean? First, the surface of the lotus leaf is hydrophobic (water-repellent). The hydrophobicity of a surface can be quantified as the contact angle between the surface and the drop of water: the higher the contact angle, the more hydrophobic the surface (see image to the right). Surfaces with a contact angle <90° are called hydrophilic (‘water-loving’); those with a contact angle >90°, hydrophobic. Some plants, which are known as superhydrophobic, achieve contact angles of up to 160°, with only 2-3% of the drop’s surface in contact with the surface of the leaf. However, the lotus leaf is not only superhydrophobic but also covered in the aforementioned waxy bumps. These reduce the contact surface of the water droplet still further (imagine the drop sitting on the points of the bumps), with the result that the drop barely touches the leaf (only 0.6% surface contact) and runs off easily.

We have seen how the water flows off, but how do the leaves get rid of dirt particles? Plants are exposed to a variety of pollutants, many of which are inorganic (dust or soot), although others are of biological origin (e.g. fungal spores, conidia, honeydew or algae). On unwettable leaves such as those of the lotus, it is not only the adhesion of water to the surface that is reduced – dirt is also simply washed away by the water. This is not as obvious as it seems, though: there are two different types of dirt particles – hydrophilic and hydrophobic ones. Hydrophilic particles such as silt are taken up in the water droplet and cannot escape again. A track is visible on the leaf where the drops have taken up the particles and washed them away.

But what about the hydrophobic particles? You might expect them to stick to the hydrophobic leaf surface, but in fact a drop of water will remove these particles too. How does this happen? The particles only touch the outermost tips of the wax structures, and since this contact surface is tiny, so are the adhesive forces between particle and leaf. They are so low that even the small adhesive forces between the water-repellent particle and water are stronger. So unlike the hydrophilic particle, which is taken into the drop, the hydrophobic particle sticks to the surface of the drop. The end effect is the same, though – it is washed off the plant.

Superhydrophobicity is not limited to lotus plants: other plants have self-cleaning properties thanks to mats of hair covering their leaves, butterfly wings have tiny ratchets that direct raindrops away from the insect’s body no matter whether the wing is tilted up or down, and the cuticle of dung beetles has embossed geometrical textures that make it hydrophobic.

What is the advantage of these self-cleaning surfaces? For sedentary plants, they are a way of protecting themselves against micro-organisms, such as fungi, bacteria or algae. Most plants fight these enemies chemically through a large range of secondary metabolites, but preventing them from settling in the first place is potentially even more effective. In addition, if leaves are covered in dirt, this reduces the surface available for photosynthesis. The self-cleaning wings of butterflies have the advantage of not retaining water and becoming heavy, which would prevent the insects from flying.

Humans have developed many types of technology that emulate these hydrophobic effects. Impregnation sprays, for example, cover the material with a wax-like coating that makes the materials hydrophobic. Some façade paints go even further, forming tiny bumps when they dry. These bumps are as water-repellent as the lotus’s wax structures with the result that the painted surface becomes superhydrophobic.

Acknowledgements

The two activities in this article were developed in the context of a workshop led by Astrid Wonisch for students of the Karl-Franzens-Universität Graz who were training to become biology teachers, and included in the 2008 exhibition Naturwissenschaft und Technik zum Angreifen.

Biology students Steffen Böhm and Karin Edlinger worked together with students from the combined 7th and 8th grade (aged 17-19) biomimetics class at the secondary school BG / BRG Petersgasse, Graz, a secondary school with a science and maths focus, and the school’s biology teachers, Renate Rovan and Ruth Unger, on ‘Adhesion on flat surfaces: negative pressure’.

Biology students Anna Freudenschuss and Ingo Fuchs worked together with students from a 4th grade (aged 14-15) class at the secondary school BG / BRG Fürstenfeld, Fürstenfeld, on ‘Self-cleaning effects: hydrophobicity in nature’.

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