Exploring Van der Waals Setae Adhesive

[SkepticJ]

I've been highly interested in the van der Waals setae adhesive since it was discovered how geckos can stick to walls about three years ago. I have a few questions about it though and I haven't found the answers.

What does "directional adhesive" mean?

Do geckos have to hang from their toes or can their toes also be below them and they push up from them?

Basically I want to find out how geckos and eventually machines will be able to hold themselves on walls, buildings, cliffs etc. Could they stand off the wall without the adhesive pealing off or will they have to keep their bodies close to the surface being climbed to keep gravity from peeling them off like a long shaft of tungsten duct taped to the wall? Or does this even happen? I've never had a long tungsten rod to test it out with.

Here's a link for the people not in the know to get up to speed.
Autumn Gecko Lab

Thank you in advance for the help.

 

[SkepticJ]

Please someone help.

 

[enigma]

SkepticJ, I'm going to move this to Biology where you may get a better response.

 

[SkepticJ]

SkepticJ, I'm going to move this to Biology where you may get a better response.

Thanks. If no one does post I might just have to e-mail them. I didn't want to waste http://polypedal.berkeley.edu/Profs_office/Full/Full_home or other people's time. They have important work to do.

 

[SkepticJ]

Bumped because I still want to know.

 

[Moonbear]

I think all of your questions were answered in the website you linked to. The angle the setae are positioned relative to the surface determines if they'll stick or not, so the geckos can both get a good sticky "grip" on the surface, and then as they tip their foot forward, can again release themselves. Interesting site there. Now I know more than I ever wanted to about how bugs, amphibians and reptiles climb walls.

 

[SkepticJ]

I think all of your questions were answered in the website you linked to. The angle the setae are positioned relative to the surface determines if they'll stick or not, so the geckos can both get a good sticky "grip" on the surface, and then as they tip their foot forward, can again release themselves. Interesting site there. Now I know more than I ever wanted to about how bugs, amphibians and reptiles climb walls.

You know you might be right. Thanks. Leave it to me to not think enough about what I read.

 

[Moonbear]

I'm still wondering why this got moved to biology though. Because it's about geckos I guess. I think we should have some fun and send it over to the physicists next since neither the chemists nor biologists have anything more to add. The engineers might find it pretty cool too. If we keep passing the thread around, someone might have more to add on it.

I admit I'm still trying to figure out how it's different from a suction cup style foot.

 

[SkepticJ]

I'm still wondering why this got moved to biology though. Because it's about geckos I guess. I think we should have some fun and send it over to the physicists next since neither the chemists nor biologists have anything more to add. The engineers might find it pretty cool too. If we keep passing the thread around, someone might have more to add on it.

I admit I'm still trying to figure out how it's different from a suction cup style foot.

Why not?

The mechanism is completely different. van der Waals forces
Suction works(but very poorly, try sticking a suction cup onto sandstone or even dusty glass) by a partial vacuum trying to suck matter into it because as the saying goes "Nature abhors a vacuum."

 

[Moonbear]

Why not?

The mechanism is completely different. van der Waals forces
Suction works(but very poorly, try sticking a suction cup onto sandstone or even dusty glass) by a partial vacuum trying to suck matter into it because as the saying goes "Nature abhors a vacuum."

LOL! Yes, I caught they were saying they were using van der Waals forces, but as they describe the feet, they sounded more like a lot of little suction cups. I was more referring to the anatomy of the feet. I know, they showed pictures, but they weren't very clear when it got down to the microscopic appearance of the setae.

 

[DocToxyn]

they weren't very clear when it got down to the microscopic appearance of the setae.

From their 2002 PNAS paper vol 99 #19, p. 12252-6 :

If van der Waals adhesion determines setal force, then geometry and not
surface chemistry should dictate the design of setae. Let us
represent an individual seta as a stalk with a bundle of terminal
tips (spatulae). If we model spatulae as cylinders, each with a
hemispherical end of radius R adhering to a surface, then given
our empirical measurements of adhesive force, we can apply a
useful theory of adhesion (Johnson–Kendall–Roberts; refs. 4
and 23) to predict R for the spatulae.

Thus the ends of the setae split to multiple units (spatulae) that end in a half-spherical tip which they then calculate has a radius of 0.13-0.16 microns. I didn't include their calculation in the quote because many objects didn't convert well, but the article is freely viewable from the site linked in the original post. They did an elegant job of confirming van der Waals as the force involved and eliminating alternative mechanisms.

 

[SkepticJ]

A way think about setae is like a human hair to small to see with the unaided eye. And the hair has split ends which are too small to see with even optical microscopes. Some have even up to a thousand "split ends" per hair. The ends of the split ends have a paddle-like structure with gives good surface area contact. Hence the name spatulae, like the kitchen tool for mixing cake stuff.