Lecture 01 Motivation and Biomimetics Fall 2009

Lecture 01 Motivation and Biomimetics Fall 2009 -...

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Biophysical Chemistry 1.3 Motivation and Biomimetics
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Climbing on glass
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Adherence to atomically-flat surfaces (such as a silicone wafer)
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What makes Geckos such amazing “climbers”???
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Each foot bears 14,000 setae/mm 2 Each seta bears 100-1,000 spatulae
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Gecko toes: setae and van der Waals forces The toes of the gecko have attracted a lot of attention, as they adhere to a wide variety of surfaces, without the use of liquids or surface tension. Recent studies of the spatula tipped setae on gecko footpads demonstrate that the attractive forces that hold geckos to surfaces are van der Waals interactions between the finely divided setae and the surfaces themselves. Every square millimetre of a gecko's footpad contains about 14,000 hair-like setae. Each seta has a diameter of 5 micrometres. Human hair varies from 18 to 180 micrometre, so the thinnest human hair could hold at least 12 setae. Each seta is in turn tipped with between 100 and 1,000 spatulae. [2] Each spatula is 0.2 micrometres long [2] (200 billionths of a metre), or just below the wavelength of visible light. [3] These van der Waals interactions involve no fluids; in theory, a boot made of synthetic setae would adhere as easily to the surface of the International Space Station as it would to a living room wall, although adhesion varies with humidity and is dramatically reduced under water, suggesting a contribution from capillarity . [4] The setae on the feet of geckos are also self cleaning and will usually remove any clogging dirt within a few steps. [2][5] Teflon , which is specifically engineered to resist van der Waals forces, is the only known surface to which a gecko cannot stick. [6] Geckos' toes seem to be "double jointed", but this is a misnomer. Their toes actually bend in the opposite direction from our fingers and toes. This allows them to overcome the van der Waals force by peeling their toes off surfaces from the tips inward. In essence, this peeling action alters the angle of incidence between millions of individual setae and the surface, reducing the van der Waals force. Geckos' toes operate well below their full attractive capabilities for most of the time. This is because there is a great margin for error depending upon the roughness of the surface, and therefore the number of setae in contact with that surface. If a typical mature 70 g (2.5 oz) gecko had every one of its setae in contact with a surface, it would be capable of holding aloft a weight of 133 kg (290 lb): [7] each spatula can exert an adhesive force of 10 nanonewtons (0.0010 mgf ). [4] Each seta can resist 10 milligrams-force (98 μN ), which is equivalent to 10 atmospheres of pull. [2] . This means a gecko can support about eight times its weight hanging from just one toe on smooth glass.
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Lecture 01 Motivation and Biomimetics Fall 2009 -...

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