Many of us already know that adhesion is the sticking together of two substances.
In nature, one finds animals that have the ability to adhere to surfaces, being able to crawl, squirm, and climb up, down, side-to-side on various surfaces. Animals such as geckos and frogs have this ability. Insects also exhibit these characteristics, like spiders, beetles, caterpillars, etc.
These living organism's capabilities to stick to surfaces is something that our group is trying to find out. How can we, humans, mimic and scale up the gecko's adhesive characteristics?
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GECKOS
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| http://www.howstuffworks.com/spider.htm/printable |
INSECTS
Unlike geckos and spiders, insects (other than spiders, of course) secrete a fluid from the tips of their feet that have adhesive properties. Insects have either hairy (fibrillar) or smooth adhesive systems on their feet, both requiring the fluid secretion. Some would predict that the more adhesive fluid secretion, the stickier. However, multiple tests (read the packets) have proven that the effectiveness of being sticky is less than that resulting from having "just the right amount" of fluid secretion. Like too much fluid, too little is also less successful. Insects secrete the perfect of amount of this fluid to have themselves stick to whatever surface they are on.
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| http://www.physorg.com/news113486136.html |
MAJOR CONCEPTS
1. Direction-Dependence: This two word term speaks for itself. The adhesive properties of insect, gecko, frog, spider feet depend on the direction of removal. The two directions we are dealing with are proximal pulls and distal pushes. For example, imagine a person holding a gecko and pulling downward while it sticks to some surface. The gecko's uses its adhesive properties to keep itself from being pulled off the surface. The pulling downward is a proximal pull. If the gecko was pushed in the opposite direction, that would be a distal push.
Several tests have been done that have confirmed that proximal pulls exhibit greater adhesive forces than distal pushes. For hairy systems, during a proximal pull, the contact area between the adhesive and surface increased, but with smooth systems, it decreased. During a distal push for both systems, contact area decreased rapidly.
Now imagine a gecko walking up a vertical surface, like a tree. As gravity pushes down on the gecko's body (as though the gecko is being pulled downward), the adhesive forces of its feet prevent it from falling off the tree trunk. For the gecko to move upward, it needs to be able to easily remove its foot and stick it back onto the trunk for the next step. Therefore, a distal movement is made, which, as stated before, allows for rapid decrease in contact area and a low amount of force needed to carry out the step upward.
Clearly, direction-dependence is a significant idea to understand because it is a great factor to the gecko's ability to successfully stick to and move on vertical surfaces.
OUR FOCUS
However, what WAB is focusing on is not the fluid-secreting insects, but rather the dry adhesive pads of spiders, caterpillars, and geckos.
