Half a croissant, on a plate, with a sign in front of it saying '50c'
h a l f b a k e r y
"Not baked goods, Professor; baked bads!" -- The Tick

idea: add, search, annotate, link, view, overview, recent, by name, random

meta: news, help, about, links, report a problem

account: browse anonymously, or get an account and write.

user:
pass:
register,


                                                                                                                                                                                                                                                 

Pond Skater exoskeleton

  (+12, -1)(+12, -1)
(+12, -1)
  [vote for,
against]

Pond Skaters are those little insects which run around on the surface of ponds, supported by the surface tension. If a Pond Skater weighs 0.5 gram and spreads its weight over four legs and 1cm² of the pond's surface, then it logically follows that a 25kg exoskeleton with 800,000 legs spread out over 20m² of the pond would be able to carry a 75kg person.
hippo, Apr 12 2012

Pond skater http://a-z-animals....nimals/pond-skater/
[hippo, Apr 12 2012]

Artificial pondskater http://www.nature.c...6/full/445373a.html
[MaxwellBuchanan, Apr 12 2012]

Gizmag: Pondskater inspired material can float 1000X weight. http://www.gizmag.c...ater-strider/21948/
"the fibers allows the aerogel to float using the same principles employed by the water strider’s long, skinny feet. Those feet are covered in tiny hairs that trap air, and that help spread the insect’s weight across the water, keeping it from breaking the surface tension" [rcarty, Apr 12 2012]

The Hydrodynamics of Water-walking Insects and Spiders http://euclid.trentu.ca/mascot/Hu06.pdf
[spidermother, Apr 13 2012]

Leonardo Walking on Water http://en.wikipedia...alking_on_water.JPG
ca. 1490 [csea, Apr 15 2012]

Ninja walking-in-water shoes http://www.youtube....outube_gdata_player
[Ling, Apr 15 2012]

BBC News - Pond skaters' feet inspire buoyant new material [2012 Mar 26] http://www.bbc.com/...technology-17510782
"Early tests based on a cellulose aerogel constructed to a design suggested by the feet of the pond skater have revealed how buoyant it is. They speculate that a 500g chunk of the material could support five standard household fridges weighing about half a tonne." [Inyuki, Apr 16 2012]

http://web.media.mi...m/ass1/observe.html [FlyingToaster, Apr 18 2012]

not quite rule 34 http://www.bbc.co.uk/nature/17939390
[pertinax, May 06 2012]

[link]






       I am very much in favour of this idea, particularly if it heralds the coming of a new olympic event.
calum, Apr 12 2012
  

       You're thinking of pond skaters, or something. Water boatmen swim underwater.
spidermother, Apr 12 2012
  

       I like this a lot. However, I suspect that the weight of a pond skater (as noted - not a water boatman) is closer to 10mg than 1g.
MaxwellBuchanan, Apr 12 2012
  

       [spider] Ah yes, I did mean Pond Skater - the idea name has now been changed.
[Max-B] The link I've added suggests that they can weigh up to 0.5g so I've taken this as the weight and amended the numbers in the idea.
hippo, Apr 12 2012
  

       Fair enough. Also, you might not need such a large area of exoskeleton.   

       The "dimples" caused by the pondskater's feet are perhaps 2mm in diameter. For the effect to work well, these dimples must not overlap. They support the pondskater's weight by locally stretching the water surface: this causes an increase in surface area and hence surface energy, and this energy exactly offsets the energy of lowering the pondskater by a fraction of a millimetre, where it stabilises.   

       The pondskater's feet are much more than 2mm apart, but they needn't be. They could be, say, 3mm apart, in which case 9 square millimetres gives you 1/6th of 0.5g, or roughly 0.09g of support.   

       Therefore, a 100kg weight (person plus suit) would require 100,000/0.09, or about 1,000,000 square millimetres, or 1 square metre.
MaxwellBuchanan, Apr 12 2012
  

       That can't possibly be right, [Max]. A 1m by 1m square (say), supporting 100 kg, is going to sink to an average depth of about 10 cm, regardless of whether the bottom is covered in pond skater legs or simply flat. There's no net lift provided by surface tension within that square - it all just cancels out, leaving you with the tiny lift around the edge.
spidermother, Apr 12 2012
  

       Shit. I'm gonna need more legs and a bigger exoskeleton.
UnaBubba, Apr 12 2012
  

       It can be right. Consider the following thought experiment and tell me where it fails:   

       1) Take a single pondskater with a body mass of 0.5 grams. It is self-supporting on the water.   

       2) Move its legs closer together, until each "dimple" on the water surface is just clear of each other "dimple" (ie, the water between the dimples is still flat). The dimples remain the same, hence they provide the same increase in surface area, and hence the same lift.   

       3) Put another similar pondskater next to it, again just leaving clear water between their respective "dimples". Both pondskaters still float.   

       4) Continue until you have a square metre of pondskaters.   

       Again, to emphasize, the lift comes entirely from the increase in surface energy associated with each dimple. This increase in surface energy exactly balances the decrease in gravitational potential energy of the pondskater as it sinks to create the dimples. If it sank any further, the increase in surface area (of the deepened dimple) would exceed the decrease in potential energy - that's why it sinks no further.   

       As long as the dimples aren't close enough to merge (reducing the additional surface area), you can have as many dimples as you like and each will contribute similar lift.
MaxwellBuchanan, Apr 12 2012
  

       Yes, there is a change in free energy at the water surface, which tends to flatten it, so countering the force of a foot. But that force is also entirely transmitted into the body of the water, where it is encountered by ordinary buoyant forces. In other words, the rules of buoyancy still apply. I think you'll find that the volume inside the dimple* corresponds to a volume of water massing the same as the mass being supported.   

       A 100 kg mass still has to displace 100 kg of water to float. A miniscule proportion of that volume will be that contained within the meniscus* around the edge of your square metre, but the rest is ordinary displacement. The difference between with the feet and without is like the difference between sitting on a bed of nails in a rubber bottomed boat nail side down versus nail side up - i.e. no difference.   

       * When I say "within" the meniscus / dimple I mean the the space enclosed by the plane of the mean water surface and the dimple's surface. Those spaces act like little boats, the surface tension being like a stretchy rubber hull.   

       In fact, I think that your version implies perpetual motion. Put scater feet on one side of a boat, and not on the other, and off you go! I have inclings of a marvellous proof of this, which alas this brain is too fuddled right now to contain.
spidermother, Apr 12 2012
  

       To put it another way, what's holding the water _up_ (against the surface energy gradient) in the regions between your closely spaced feet?
spidermother, Apr 12 2012
  

       //A 100 kg mass still has to displace 100 kg of water to float// - to float, yes, but not if it's being supported by surface tension. Another thought experiment - the amount by which a trampoline sinks when someone stands on it depends on the tension of the trampoline as well as the weight of the person - you wouldn't say that any trampoline has to deform a certain amount for a given weight person.
hippo, Apr 12 2012
  

       No, but the force on the trampoline skin is transmitted to the frame. There is no equivalent here of a frame, unless the pond is very very tiny and its banks are hydrophylic. The only place the forces can be transmitted is to the water, and the only mechanism is hydrostatic pressure - i.e., buoyancy. The idea that multiple closely spaced dimples can create a net upward force due to surface tension is to propose the equivalent of skyhooks.   

       Instead, there is an equal-and-opposite force of 980 newtons between the feet _and the water_, which corresponds to a pressure of 980 pascals, as found at a depth of 0.1 metres.   

       Your trampoline analogy would suggest that you would not sink as low if you wore cleats - because the tightly stretched skin around each cleat would create an upward force. Clearly, that's not the case. Something would have to be pushing up on the skin between the cleats to make them dome up, and to make the forces balance. What does that in the water example is hydrostatic pressure. That's why you would sink until you displaced 100 kg of water - albeit the water on the bottom is kind of wobbly. But, as I said, the wobbles and the supported weight and the pressure all cancel out.   

       Those feet are widely spaced, even on such a tiny insect, for a reason. Mess around with floating small objects on surface tension, and get a feel for what it would be like to float 100 kg/m². Try very small stones. Or churches. Lead! Lead! (What was the question again?)
spidermother, Apr 12 2012
  

       Interesting - I'm not sure. So you're saying that if you had a 20m² pond with 200,000 0.5g Pond Skaters on it, they'd sink?
hippo, Apr 12 2012
  

       I'm not sure on this one. I would say that the volume "missing" from the dimples around the pondskater's feet is smaller than the volume of its body (which will have a density of around one). If so, this kills the displacement theory. But I can't be sure.
MaxwellBuchanan, Apr 12 2012
  

       "useful"?!
hippo, Apr 12 2012
  

       OK, after some pondering, [spidermother] is right.   

       The correct analogy is not with a trampoline, but with a water surface with a stiffish rubber skin floating on it. When you stand on the skin, you sink in; the skin means that sinking by a given amount displaces more water than you would if the skin weren't there.   

       In effect, the surface tension is working like the hull of a boat, displacing a large volume of water for a small amount of sinkage.
MaxwellBuchanan, Apr 12 2012
  

       well if Max's equations were right'ish then I don't see why you can't simply have a huge dimple that's made up of many smaller dimples.   

       ie: a slightly-larger-than-a-metre dimple, say 20cm deep in the middle, formed by a whole bunch of regular dimples. The ones closer to the edge would have a bit of a horizontal component to keep the water back, as well as the vertical one that holds up the contraption against gravity
FlyingToaster, Apr 12 2012
  

       Then, once you're done running the equations, you want to figure out: If there's several people standing on a pond via surface tension, and then one of them pours some soap into the middle of the group, does the surface tension suddenly snatch them all toward the bank away from the soap, or just drop them all in the water?   

       (I'm with [calum] on wanting an Olympic sport - and I think the soapyness of a paintball's contents might just do it)
lurch, Apr 12 2012
  

       what you're saying is that the entire mass of water displaced from 280kg of weight would be .5kg ?   

       <contemplates surface tension bucket>
FlyingToaster, Apr 12 2012
  

       Now I remember why I love this place.
UnaBubba, Apr 12 2012
  

       A duck!   

       Ducks do exactly this. They have fine, hydrophobic feathers, like millions of pond skater foot hairs, which create millions of little curved menisci. This keeps the bulk of the feathers dry. So a duck is supported entirely by the pond skater effect, but it also displaces its own mass of water, and is supported entirely by buoyancy.   

       There is no paradox or contradiction. A ship is entirely supported by the tensile and compressive forces in the hull, but also entirely supported by buoyancy.   

       The reason I'm confident of this is that the slope of the meniscus rapidly approaches zero as you move away from the pond skater foot or whatever - therefore the net force imparted by surface tension alone is approximately zero.
spidermother, Apr 12 2012
  

       Are you implying that I'm deliberately trying to ignore surface tension to make the answer come out my way? Well, I'm not. Surface tension can create huge pressures - but it can't make you walk on water. The forces still have to balance. On a large scale, the way in which (say) a large flat board covered in pond skater feet pushes on the water is exactly equivalent to the way in which a plain board does. The hydrostatic forces still have to balance. Otherwise, there would be a magical coating that significantly reduces the draft of ships all on its own - which is clearly an impossibility.   

       With all due respect, your analysis up there ^ is incorrect in every single detail, so you may be arguing about a topic that you don't understand very well.
spidermother, Apr 12 2012
  

       I'm kinda confused about just how hydrophobic are the duck's feet?   

       Surely that bit of the duck not covered in hydrophobic feathers is buoyed by displacement rather than surface tension?   

       Also, given that a breach of surface tension results in an (albeit minor) catastrophic failure of the surface tension effect, then surely more of the duck's mass is then supported by displacement than it is by surface tension?
UnaBubba, Apr 12 2012
  

       All of the duck's weight is supported by buoyancy / displacement. This acts as ordinary hydrostatic pressure; more directly in the case of the feet, via the little curved menisci in the case of the feathers. Surface tension merely prevents the water from passing into the (mostly air-filled) bulk of the feathers; it acts like a waterproof skin. The situation would not be radically changed if you gave the duck a little pair of rubber or GoreTex waders.   

       If the surface tension is breached, and the feathers become saturated, it's just like flooding the buoyancy tanks on a submarine. The duck will still be supported entirely by buoyancy, but it will sit much lower in the water, since its effective density has increased.   

       So, it doesn't really matter whether the duck's feet are hydrophobic. They just displace their own volume, and provide the equivalent amount of buoyancy, either way.
spidermother, Apr 12 2012
  

       //According to [hippo] and [myself] its approximately 15kg per square metre.//   

       Ahh! kg per square metre? Please use proper units.   

       But that calculation is entirely meaningless; it depends wildly on the shape of the meniscus. Surface tension can support many megapascals (hundreds of tonnes per square metre, if you like) quite easily. Real world examples include GoreTex and the water-air interface in tree leaves. But so can a sheet of steel. Whether the flow of water is prevented by an unbroken barrier, or by many curved menisci, makes no difference to the fact that a large mass, on a large body of water, is supported entirely by buoyancy.   

       You seem to be seeing a phantom inconsistency in what I'm saying. There's no need to make up my mind. It's like the question of whether my weight is supported by the skin of my feet or the soles of my shoes. The answer, of course, is both. A duck is supported by (a) buoyancy, (b) surface tension, (c) the elastic strain in its feathers, and (d) all of the above.
spidermother, Apr 12 2012
  

       That is just simply incorrect. There is no such break point. Again, surface tension can support millions of pascals per square metre (but only on a tiny scale), or micropascals per square metre.
spidermother, Apr 12 2012
  

       //So objects denser than water always sink ?// No. But objects that float on a large body of water always do so by displacing their own mass.   

       Take the classic razor blade trick. It's quite easy to float a razor blade on the surface of water (I just went and did it). But if you look carefully, the surface of the metal is below the free water surface. The meniscus goes up and out from the edges of the blade, like the hull of a little boat, with the blade lying on the bottom. The total volume of water displaced exceeds the volume of the steel, but masses the same as the steel.
spidermother, Apr 12 2012
  

       Please be careful what you say. You are, in fact, mistaken. It is indeed a question of scale. Your //150 Newton per metre// is entirely arbitrary. Again, a very small meniscus can support vastly more than that. A large one can support vastly less. Why do you think water goes further up narrow capillary tubes than wide ones? Scale, scale, scale. You simply can't extrapolate from a water strider to a vessel of several square metres.
spidermother, Apr 12 2012
  

       Seriously, stop being such a fuckwit. I've never denied the existence or the importance of the skin effect in the flotation of a pond strider.   

       And your 20 square metres is the result of arbitrary parameters and flawed reasoning.
spidermother, Apr 12 2012
  

       Let me make some sense of this.   

       If there is 3.5 billion spread legs in the world and habitable surface area of 75 million^2 killometers, then there should be about 45 possible sexual encounters per square kilometer, or one every 20 meters in any direction. I don't get laid nearly that often either.   

       For a silly reducio absurdum idea, this sure has generated quite a bit of semi-serious discussion.
rcarty, Apr 12 2012
  

       Ducks taste better than water striders.
UnaBubba, Apr 12 2012
  

       I'm pretty sure a water strider would float dead on its back.
rcarty, Apr 12 2012
  

       I just learned more in five minutes...
not sure just what I learned yet but I'm betting it was something profound.
  

       Yep. Houston, we have tickle.   

       [pondskate^H^H^H^H spidermother] "Again, surface tension can support millions of pascals per square metre (but only on a tiny scale), or micropascals per square metre." - so you're saying that a 20m² pond wouldn't be able to support 200,000 Pond Skaters?
hippo, Apr 13 2012
  

       //They have fine, hydrophobic feathers,//
Rabid duck feathers?
AbsintheWithoutLeave, Apr 13 2012
  

       hmmm - how about an similar exoskeleton for walking around on ceilings, based on the way houseflies are able to walk upside down on ceilings?
hippo, Apr 13 2012
  

       When I was young we all used to call them “water boatmen” so there. I demand the title be changed back.
pocmloc, Apr 13 2012
  

       "In a static situation, (2.9) yields a generalized form of Archimedes principle: the force on a static floating body is equal to the weight of the fluid displaced. Mansfield et al. (1997) and Keller (1998) showed that the magnitudes of the buoyancy and curvature forces on a floating body are equal to the weights of the fluid displaced by the meniscus, respectively, in- and outside the line of tangency C"   

       Source: "The Hydrodynamics of Water-walking Insects and Spiders"(2006), by David Lite Hu, B.S., M.I.T., 2001. (linked)
spidermother, Apr 13 2012
  

       [spidermother] …and from the same link: "15th century ninjas developed flotation shoes called “Mizugo,” or water spiders, to be used for walking stealthily on the water surface."
hippo, Apr 13 2012
  

       Also from the same article: "A man of mass M = 70 kg would require feet of perimeter P = Mg/sigma ~10 km to be supported by surface tension."   

       Those ninja devices rely on kinetic forces - like a dolphin doing a tail-stand - not surface tension. The article explains that quite clearly.   

       And even when surface tension is the dominant effect (such as with a water strider) a volume of water is displaced of weight equal to that of the supported object, as that article spells out in detail. Which is what I've been saying all along.
spidermother, Apr 13 2012
  

       //A man of mass M = 70 kg would require feet of perimeter P = Mg/sigma ~10 km to be supported by surface tension//   

       Now that, I guarantee, is bollocks.   

       Yes, I agree with [spidermother]'s point that the bouyancy comes from the displacement of water. And the displacement of water comes mainly from the "dimpling" around the feet which, in turn is a result of the rubber-sheet-like effect of surface tension.   

       But even allowing all that, 10km is ridiculous. A person needs to displace 100kg of water. Assuming that an array of hydrophobic footlets can dimple the water so that it's *average* height is lowered by only 0.1mm, then the mass of water displaced by the footlet dimples (en masse) is 100 grams per square metre. Hence, a platform with a total area of about 1000m2, and having an underside covered in pondskater feet, will displace 100kg of water through surface-tension- dimples alone.   

       I can only assume that their "10km perimeter" applies to a single, flat hydrophobic foot.
MaxwellBuchanan, Apr 13 2012
  

       'Perimeter' means 'perimeter'. A 10 km perimeter can be achieved with a square 2.5 km across, or with a million squares 2.5 mm across - which could fit into a smaller total area.
spidermother, Apr 13 2012
  

       Yep, I've checked his calculations, and 10 km is correct. It comes straight from the definition of surface tension, assuming that the meniscus is vertical at the line of contact - the best-case scenario, and the point at which the slightest increase in weight will cause instant sinking.
spidermother, Apr 13 2012
  

       //And the displacement of water comes mainly from the "dimpling" around the feet which, in turn is a result of the rubber-sheet-like effect of surface tension.// To be precise, it comes from two places:   

       1) the contact area of the foot on the water, and   

       2) the curved meniscus.   

       In the case of the water strider, 1) is much smaller than 2). For objects of increasing size, 2) becomes insignificant.
spidermother, Apr 13 2012
  

       Yes, obviously.
MaxwellBuchanan, Apr 13 2012
  

       unless you're a ninja.
hippo, Apr 13 2012
  

       [bigs], even something supported by surface tension is, ultimately, supported by bouyancy. The surface tension causes the leg to create a "dimple", and it's this dimple which (since it's displaced water) gives the bouyancy. It's as if the water skater had little dimple-shaped boats on its feet.
MaxwellBuchanan, Apr 13 2012
  

       I admire your tenacity, at least. As a reward, here's a scenario in which an object can be supported by surface tension in a trampoline-like manner, and in which buoyancy is not important.   

       Place a capillary tube vertically in a container of water, as in the classic demonstration of capillary action. The meniscus will ascend, until the change in surface energy at the glass-water interface balances the change in gravitational potential energy. Drop a tiny, heavy, hydrophobic bead down the tube. It will sit on the water surface, and the water surface will drop a bit, until the energy is again minimised. In this case, the object is supported by cohesion in the surface water molecules (like the tension in the trampoline skin), which in turn is supported by adhesion between water molecules and the glass (like the springs on the trampoline), which in turn is supported by the strength of the glass (like the frame of the trampoline) etc, and finally the force supporting the tiny bead is transmitted into the Earth and everything balances.   

       The situation in a large body of water is different. The slope of the meniscus rapidly approaches zero, even a few cm from the edge of the object. Tension in a horizontal surface transmits only horizontal forces. The only way the weight of the pond-skater (a vertical force) can be transmitted into the earth is by hydrostatic forces; i.e. Archimedes principle. So the chain of forces goes leg on surface -> tesion in steep walls of dimple -> hydrostatic forces on the underside of the dimple (buoyancy) -> pressure on the bed of the lake etc.   

       All of this is why I carefully stressed that we were dealing with a large body of water (although 'large' only means a scale above a few cm), and the importance of scale.   

       And the reason it's important in this context is that it clearly predicts that (say) a 1 m² raft carrying 100 kg will sink to an average depth of about 10 cm, *whether or not* it uses the pond skater effect.
spidermother, Apr 13 2012
  

       [bigs], [spidermother] made the point earlier that the trampoline works because the skin is connected to the frame.   

       As mentioned above, the best analogy for surface tension (but on a large scale where we can see what's happening) is a thick but flexible rubber sheet sitting on the water surface. When you stand on it, you create two dimples which are much wider than your feet, and these big dimples displace enough water to offset your weight.
MaxwellBuchanan, Apr 13 2012
  

       It would float. Your point being?   

       OK, first, assume the pepper is denser than water (otherwise there's no question).   

       Each pepper grain sinks a little, creating (thanks to the membrane-like surface tension) a little dimple. When the mass of water displaced by the dimple balances the mass of the pepper grain, it stabilises.   

       Truly, it is bouyancy. The surface tension is important because it allows the particle to create a "dimple", but it's still just bouyancy.   

       (I missed this point at the outset of this whole debatle, but [spidermother] is correct. Surface tension is the mechanism, but bouyancy is what holds up the pondskater).
MaxwellBuchanan, Apr 14 2012
  

       Yes, I get this now - the surface tension is a means to create a concavity in the pond's surface which gives buoyancy. I am impressed that [spidermother] found a PhD thesis on this. It makes me wonder whether any sufficiently contentious Halfbakery idea can be assumed to have a PhD thesis written about it.
hippo, Apr 14 2012
  

       Exactly, [hippo].   

       And yes, there's a PhD thesis on everything. I have personally seen a thesis on the microscopic structure of Mahon cheese. It is in a display case, in a Mahon cheese museum. I kid you not.
MaxwellBuchanan, Apr 14 2012
  

       "The contention singularity: a causal analysis of doctorate subject probability and the halfbaked contentiousness index"   

       We present evidence for a threshold level of contentiousness, above which the probability of the existence of a contention-resolving doctoral thesis ...
spidermother, Apr 14 2012
  

       //We//? Is this a Borg thesis?
MaxwellBuchanan, Apr 14 2012
  

       //I'm still not convinced.//   

       Oh, for goodness' sake. Why would I make up such a thing? It's in a display case in the Mahon Cheese Museum at Hort de San Patrici, on Menorca. It's more or less directly opposite the door as you go in. If you don't believe me, go and see for yourself.
MaxwellBuchanan, Apr 14 2012
  

       //so long as the legs are equally distributed// Have you ever seen a performance of Riverdance where they were not? Look closely. Not a single performer with fewer than two legs, or more than two.
mouseposture, Apr 14 2012
  

       And all neatly trimmed to precisely the length needed to reach the floor.
spidermother, Apr 14 2012
  

       Without all this maths I'll have a scaled up replica exoskeleton ready for the pond by this winter.
rcarty, Apr 14 2012
  

       //milliRiverdances// Thanks for the update; I'm still reckoning by the reel (or the old Middle Earth unit, the leg o' lass).   

       [rcarty] If you're serious about making a model, I'd start with a square of very thin balsa or expanded polystyrene, with L shaped lengths of thin plastic distributed over the bottom. The contact surface (the base of the L) needs to be a line, rather than a point.
spidermother, Apr 14 2012
  

       Having the ability to walk on water already, I find this redundant, and useless. But that's just me.
blissmiss, Apr 14 2012
  

       OK, I've made a little proof-of-concept model. I tried 3 staples with one leg cut off (a convenient way to make L shapes) mounted on a thin piece of expanded polystyrene, but it was all too heavy. I got it to work eventually with mono-filament fishing line legs, and the body cut down to a Y shape - the bare minimum to support the 3 legs. The result looks rather like a 3-legged water strider, even though that wasn't the intention; art imitating life, or convergent evolution, or something.   

       I now feel that with thinner line, and more precise construction, it would be possible to make a fairly large continuous raft supported by many feet. Supporting a human would be a very difficult task, though. You would need a very large, very stiff, and very light structure to distribute the weight.
spidermother, Apr 14 2012
  

       (Quoted in the linked thesis)   

       If one morning I walked on top of the water across the Potomac River, the headline that afternoon would read "President Can't Swim."
-Lyndon B. Johnson (1908 - 1973)
  

       (Speaking of miracles, I hope it comes to the author's attention that someone actually read his thesis. I wanted to drop him a line, but couldn't find a contact. Send an email c/o his department, I suppose.)
spidermother, Apr 14 2012
  

       Hm, walking on water... Didn't Leonardo have that designed around 1500? Ah, yes [link] Note: displacement, not surface tension, and poles for stability.   

       I also seem to recall some floaty shoes in a youtube video a while ago, but can't be bothered to find it just now.
csea, Apr 15 2012
  

       i wonder if those are the Ninja stealth shoes from Mythbusters? (link)
Ling, Apr 15 2012
  

       Although I have no objection to attempting one's'construction my previous comment was a poorly delivered joke.
rcarty, Apr 15 2012
  

       A very large, very stiff structure with hundreds (thousands) of feet would have a major problem, because it will only work if the water is perfectly flat. If there are any ripples some of the feet will dig in more than others. As one digs in, it will break the surface, losing some of its load bearing capacity, bringing more feet into contact with a heavier than normal load, causing it to break the surface, and so on and so forth, producing a catastrophic dousing.
MechE, Apr 15 2012
  

       That would be a gedunkin experiment then.
4whom, Apr 15 2012
  

       Sinking a pondsketer? That would a water insection?
UnaBubba, Apr 15 2012
  

       //ripples// Since water doesn't form discontinuities*, it should be easy (for somebody else) to build it to ride out the waves; at least for the non-loaded version.   

       * hmm, okay, let's say it doesn't have to be built to withstand breaking waves.
FlyingToaster, Apr 15 2012
  

       You'd have to figure out a way to evenly distribute the person's weight among the thousands of your little surface tension pontoons. Otherwise it would just fold up and sink like if you jumped onto a sheet of plastic floating on the water. You've got reality popping up it's ugly head with the catch 22 that anything strong enough to distribute your load would add enough weight to break the surface tension before you even put your person on it.   

       You could sort of cheat. Build a pyramid shaped platform big enough to distribute the load across the thousands or millions of insect feet you'd need and have it held up by cables designed to carry only the platform's weight, not a pound more. When the person steps on it, the additional weight would be born by the surface tension of the water so you could say you've technically floated a person with surface tension.   

       It's like an underhanded bar bet but the person you made the bet with would arguably have to pay up.
doctorremulac3, Apr 15 2012
  

       I think you'd need lasers to track the speed and amplitude of incoming ripples and some software to model the progress of these ripples as they pass under the 'legs' of this device and to move the legs up and down to accomodate the ripples.
hippo, Apr 16 2012
  

       At least. More likely it would have to be done in a completely wind proof enclosure because the slightest breeze would cause the whole thing to collapse. Plus the suspension mechanism would need to allow the 1/32" or so drop to allow the water surface to deflect slightly before the cables broke.   

       Does the person have to be alive and in one piece? Because if you broke a dead human body into 800,000 pieces and distributed it among the 800,000 legs that might work. Although at that point I'm not interested in taking part any more.
doctorremulac3, Apr 16 2012
  

       How about a hinged multiple cantilever? I'm thinking about the devices which they use to strength-test aircraft wings. They consist of a large beam, suspended from the middle. From each end of it are suspended two more beams, and from each end of those, two more... and so forth, until there are perhaps sixty-four or more hinged points of attachment to the wing being tested.
MaxwellBuchanan, Apr 16 2012
  

       Yes I thought of one of those, but didn’t know what it was called and couldn’t be arsed to type out a description.
pocmloc, Apr 16 2012
  

       I don't doubt it's possible to design a controlled aray that would compensate for the movement of water. What I doubt is that it's possible to design such an array where the control mechanism for each leg isn't greater than the weight it can support.   

       I'm not going to rule it out completely, as MEMS stuff surprises me as often as not, but even if it is possible, it will greatly increase the required number of legs to support a human weight due to the required overhead.
MechE, Apr 16 2012
  

       //What I doubt is that it's possible to design such an array where the control mechanism for each leg isn't greater than the weight it can support.//   

       You mean, like a pondskater?   

       I think in the end, it all boils down to "why"? If you made this device, it would look like some sort of big articulated doormat that sat on the water surface. Hidden underneath it would be a million tiny little leglets making a million tiny little dimples. But a slab of expanded polystyrene would do the job just as well, by god's own bouyancy alone.   

       I suppose one advantage of using surface tension rather than simple flotation is that there must be less hydrodynamic drag. So maybe there is, after all, a use for a surface-tension-supported mat.
MaxwellBuchanan, Apr 16 2012
  

       Maybe the design should just incorporate thousands of pond skaters. I mean, is it really more strange to use 200,000 pond skaters an elaborate device to support a person on a pond than it is to use a pair of horses to pull a carriage along a road?
hippo, Apr 16 2012
  

       The simplest solution is to use the elasticity of each leg to provide passive independent suspension. And the simplest way to do that is to make the legs almost exactly like pond skater legs - a sloping part above water, and a horizontal part lying on the water.   

       [edit] Or use actual pond skaters, as [hippo] said. They would need some freedom of movement, particularly in the vertical direction. I'm imagining each skater harnessed to a fine elastic lead resembling an 'invisible dog' lead.
spidermother, Apr 16 2012
  

       I can't see anyone (apart from lurch) suggesting that surface tension is easy to break down by e.g. ripples and waves and suchlike. When it does the whole thang goes underwater.
This ain't never going to float at anything other than the scale of a Pond Skater.
Or am I missing the point of the Halfbakery?
gnomethang, Apr 16 2012
  

       //I can't see anyone (apart from lurch)// Hello? That's precisely what we've all been discussing. All that stuff about springs and cantilevers and control mechanisms is as much as to say 'this would be extremely difficult to pull off'.
spidermother, Apr 16 2012
  

       //You mean, like a pondskater?//   

       Exactly. A pond skater has a mechanism which can just support itself, the power source (digestive system) and control circuitry (most of the nervous system) with very little left over (reproductive and a few bits of the nervous system). Given that a human scale device would require much more leg travel and probably have a less eficient hydrophobic coating, I stand by my statement.
MechE, Apr 16 2012
  

       I'm pretty sure that, when a gentleman pondskater and a lady pondskater become very friendly, they do not keep all twelve limbs on the water surface. This means there must be some overhead.
MaxwellBuchanan, Apr 16 2012
  

       That's covered in that thesis. All pond skaters are able to support more than twice their own weight, partly for that reason.
spidermother, Apr 16 2012
  

       Eight limbs and there's a picture in the first link.
rcarty, Apr 16 2012
  

       OK, that picture shows 10 of the 12 legs on the surface. Nonetheless, the very largest water striders weigh slightly less than half what their legs can support, and the smaller ones much less than half.
spidermother, Apr 16 2012
  

       I think the other two limbs function more like arms.
rcarty, Apr 16 2012
  

       Pond skaters use all 6 legs to support themselves. The front legs may be shorter, and are clearly being used here by the male for clasping, but they are also load bearing (as seen in the female), and used in ordinary locomotion.   

       Anyway, I'm disinclined to question Hu's reporting that the margin of safety does not exceed ½.
spidermother, Apr 16 2012
  

       Was this idea inspired by the BBC News article published exactly 20 days ago, about Pond skater-inspired super- buoyant material created by researches in Helsinki? (See link.)
Inyuki, Apr 16 2012
  

       In the context of this discussion, that material is more closely analogous to a duck's feathers than to a pond skater's feet.
spidermother, Apr 16 2012
  

       Has anyone else noticed that the argument over whether or not 800,000 water strider legs can support a full-grown man has devolved to a detailed examination of bugs fucking?
Alterother, Apr 16 2012
  

       Yes, but the more emotionally mature of us had already moved on.   

       Now, how does Rule 34 apply to pond skaters?
spidermother, Apr 16 2012
  

       I moved on because sexual interest diminishes after climax.
rcarty, Apr 16 2012
  

       Damn, I'm now thinking about crotchless rubber pond skater suits. Yeah baby, spread those limbs, distribute those dimples.
spidermother, Apr 16 2012
  

       Oh my. Or erggghhh. Uhm ugh icky.
blissmiss, Apr 16 2012
  

       ^ But [miss], they started it!
spidermother, Apr 16 2012
  

       You got there first.
UnaBubba, Apr 16 2012
  

       [Bubba] wins.
blissmiss, Apr 16 2012
  

       //[Bubba] wins//, because I //got there first//. Can't argue with that.
spidermother, Apr 16 2012
  

       No you could, but there is that whole idol, love-fest going on...FOR NOW!
blissmiss, Apr 16 2012
  

       No, I most emphatically CAN'T argue with that because ... OK, you win.
spidermother, Apr 16 2012
  

       But I don't wanna win, I wanna lose cause that gives me the sympathy vote, which means I'll win.
blissmiss, Apr 16 2012
  

       Good, good! More material for my thesis!
Alterother, Apr 17 2012
  

       [Inyuki] No - I hadn't seen that. Interesting though.
hippo, Apr 18 2012
  

       [hippo], yes, cause it supports your idea!
Inyuki, Apr 18 2012
  

       //more material for my thesis// baked. <link>
FlyingToaster, Apr 18 2012
  

       That's not what my thesis is about.
Alterother, Apr 18 2012
  

       Boy, [FT], I hope that paper was marked down for the speliing errors!
gnomethang, Apr 19 2012
  

       The exoskeleton structure has to, somehow, evenly vector the 100kg to each dimple.
wjt, May 06 2012
  

       is that a European or an African pondskater?
not_morrison_rm, May 07 2012
  

       It couldn't be African, African pond-skaters are non- migratory.
Alterother, May 07 2012
  

       I'm still not unconvinced.
not_morrison_rm, May 10 2012
  
      
[annotate]
  


 

back: main index

business  computer  culture  fashion  food  halfbakery  home  other  product  public  science  sport  vehicle