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I want to measure friction coefficient between a beetle and an inclined plane (angle of inclination $\theta$). As we all know, friction coefficient $\mu$ is given with: $\mu=tg \rho$, where $\rho$ is the angle of sliding down monotonically. Is it sufficient to just put a beetle on the plane and slowly increase $\theta$ and see at which angle will it slide down? Or do I need to take into account something more? Is my experiment too simple?

I'm asking because I have seen some beetles trapped in a glass bottle. I would like to know more about beetle's possibilities to escape from such trap. I want to know about how should be a bottle shaped and what is the best material to make escape easy for the beetle.

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  • $\begingroup$ What the angle limit will measure is the static friction, i.e. how much force it takes to "unstick" the beetle. Once the beetle is sliding, the kinetic friction will kick in, which is usually smaller than the static friction. You also have to be aware that both depend on the material of the plane. Are you planning different experiments with different beetles and materials? $\endgroup$ – CuriousOne Sep 22 '15 at 20:50
  • $\begingroup$ @CuriousOne I just want to know how should be a glass bottle shaped to allow a beetle to escape from it in as many bottle's positions as possible. $\endgroup$ – user46147 Sep 22 '15 at 21:04
  • $\begingroup$ I see. Most importantly for the shape there can't be any overhangs and the angles should be shallow, like in a bowl. I can tell you from observations that I have seen insects and spiders trapped by very shallow slippery surfaces, though, so changing just the shape of a bottle won't help much. What I usually do to allow them to escape (without having to be there myself to help) is to hang a piece of rough paper along the side of the container all the way to the top, that way the critters have a non-slippery surface to hold on to and they usually find their way out of the trap by themselves. $\endgroup$ – CuriousOne Sep 22 '15 at 21:11
  • $\begingroup$ @CuriousOne I'm also thinking about bottle's material. It would be fun if bottles were made of glass that would be "frictionfull" enough to allow insects escape. This means I should measure "the friction of insect walking" on different glasses. $\endgroup$ – user46147 Sep 22 '15 at 21:16
  • $\begingroup$ The friction with glass is mostly given by the surface roughness, but even rough glass may not be enough for insects to hold on to. Their feet are made for natural materials and they usually have small hooks and suckers that help them to stick to leaves etc.. Those don't work well on glass. Give it a try and see how the beetles are doing. You will learn quickly what works and what doesn't. $\endgroup$ – CuriousOne Sep 22 '15 at 21:25
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The experiment you propose seems fairly sound, but you would need to test it many times to check whether a consistent co-efficient of friction can describe a beetle's stickyness well. With small creatures, friction is often way too blunt a concept to measure or describe a their ability to cling to a surface. Some creatures can switch their clinging ability on or off at will, by forming temporary van der Waals bonds with the surface although I'm not sure whether any coleoptera (the beetle order: aptly named the "sheathed wings") can do this: you'd either need to ask an entymologist or try the experiment to see. Mostly creatures who do this have small hairs on their feet to exploit van der Waals forces for attachment, and some can switch between an effective infinite co-efficient of friction (i.e. you can't unstick the creature without ripping it apart) and zero co-efficient almost instantly. Actually, this is not friction at all since the reaction force from the surface is towards the surface and the creature's presence upside down on the surface puts the surface is a state of tensile rather than compressive stress, as in a friction scenario. The geckos are the most famous users of the van der Waal force to afford an at-will, dynamic sticking ability and their sticking organs (kitted with nanometer scale special hairs called setae) can be seen in the photo below (source: Wikipedia "Gecko" article). But there are also many arthropods who can do this as well: flies and spiders come to mind.

Gecko foot Micrometer and nanometer scale gecko feet structures Uroplatus fimbriatus on glass

Moreover, in physics when you get down to insect size and smaller, often everyday physical intuitions don't work so well. The importance of "cold welding" type phenomena and van der Waals forces for very light, small things makes friction nonuseful for the description of small being statics.

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