This experiment was inspired by the "robots walking on water" from Carnegie Mellon University (CMU).
CMU's robots are called STRIDE, for Surface Tension based Robotic Insect Dynamic Explorer. They mimic the way water strider insects "walk" on water.

I wanted to see how difficult it is to make a "robot" that moves on top of the water surface, purely based on surface tension. I found out that recreating the principle is not hard at all.

When carefully putting the construction on the water, the surface tension keeps it on top. When disturbed, the construction sinks, proving it is really the surface tension that does the trick.

The vibration of pager motor gives it some rudimentary propulsion (both on land and on water actually).

I hope this experiment inspires you and I look forward to hear what you can do with it. And of course, your vote is welcome.

Step 1: Materials

You need a material that is light and stiff, somewhat hydrophobic and available in shapes giving a lot of "contact edge".
A test showed that the small diameter carbon fiber reinforced rods work fine. You can find these "carbon" rods at modeling shops stocking indoor flying materials. I used two 0.8mm diameter rods, about 1 m in length. The more commonly available 1 mm diameter rods are probably also worth a try.

The other materials used are:
A small a pager motor
A small 1.5V button cell battery
Some scotch tape
A tiny piece of double sided tape

Step 2: Basic Construction

The rods are bent and kept in shape by attaching them to each other with ordinary scotch tape. At the center "node" I made support with some more scotch tape, keeping motor and battery out of the water. The motor was fixed with a tiny piece double side tape, making sure the excenter weight turns freely. The motor leads had their electric insulation removed over a couple of mm and each one is taped to one pole of the battery. I learned this method from the "Evil Mad Scientist BristleBot".

Step 3: Further Devellopment

In a first attempt to improve the propulsion somewhat I added som some small scotch tape "fins" to the back "legs". This gives indeed a slightly more noticeable forward movement when testing in the pool. This can probably be improved by repositioning the motor on a "leg" rather than on an "node". But to keep it on top, this needs a more elaborate fixing of the motor than what I had available at the poolside. This also needs some testing in order not to upset the weight distribution to much. Something for a next attempt.

With the nice ripple pattern generated, the construction makes a simple kinetic art bot. In a next stage, the principle could form the basis for an aquatic BEAM, by adding some sensory input controlling two differently oriented pager motors. You can compensate for the extra weight (we are talking of grams of course) by adding extra carbon fiber rods.
Rather than using cellotape to secure the joints, can I use solder instead?
Not unless you use metal rods instead of carbon rods. Carbon (carbon fibre reinforced epoxy that is) can not be soldered. You might use glue, but as there is not much surface to glue, a cellotape joint will be more resilient.<br><br>As for metal rods, these will be rather heavy, and solder will be heavier than tape. You can try, but I really recommend carbon rods of maximum 1mm diameter, available at hobby shops selling RC airplane kits and probably at kite shops to. if you can get 0.8 or 0.6 mm, this will be even better.<br>
<p>Couldn't I just use JB weld (epoxy) and clamp it all together for a stronger finish?</p>
<p>There is simply no need. The tape give gives an immediate hold, is strong enough and lasts a long time.</p><p>Also it is very light. There is not a lot of surface contact, so using something like epoxy would mean adding some glue draped around the rods, adding weight.</p><p>If you really want to ad strength, I would recommend wrapping around some sowing thread and soak that wit superglue. You could use carbon fibre thread and/or thin epoxy instead, but that would probably be overkill.</p>
Thanks for the fast reply, helps a lot. Haven't actually started this project though, but hoping to soon :)
I LOVE IT!!!!!!!! must make it sometime. Keep it up!!!!!1
Sweet ive gotta try this!
amazing!!! however, does the motor short out after it sinks?<br />
No it doesn't, some current leaks through the water, but that is far from shorting out&nbsp;<br /> <br /> for mor info on low voltage motors and water, see: &nbsp;<a href="https://www.instructables.com/id/Simple-solar-powered-pocket-submarine-with-depth-c/" rel="nofollow">https://www.instructables.com/id/Simple-solar-powered-pocket-submarine-with-depth-c/</a>
ahhh... thank you!<br />
I wonder if having the motor hooked up to a small air propeller might be more propulsive without disturbing the water and sinking it?<br />
It might be tricky, but it could work.<br /> The motor would need to be mounted on some pillar and a&nbsp;problem might be the&nbsp;pillar acting as a lever and the propulsion force pushing the front of the construction down.
you can do the exact same thing with a paperclip, much harder to get it to stay on top, but once it does its really cool
Indeed you can with thin paperclips, and the effect is cool indeed. However as it is hard to get it to stat on top, it is clear there is no room for any propulsion. The flat metal/plastic strips used to close garbage bags or to keep electronics cables together when you by them are easier to get to stay on top. But you can not upscale it to be able to cary any propulsion. Even if you have longer strips, they are not stiff enough.
can thin silver metal wire work? its 26 gauge wire
I doubt it. Silver does not really have an interesting stiffness/weight ratio, but you can easily test it by trying if it stays on the surface of the water. First just by itself, then with a tiny extra weight.
well its not really silver par say its silver coloured steel wire, or ... what if i were to heat it up till it glows then plunge it into cold water to make it stiff?
Steel has a better stiffness/weight ratio than silver, but still worse than carbon fibre composite. Heating and quenching can increase hardness and to some extent strength, but it will not change the stiffness (elastic modulus). The alloy determines the stiffness.
dang...how bout fibre glass strips impregnated with resin then formed to the shape
That would probably work. But if you go into that effort it would be better to use carbon fibre.
Uhm...I am Having Trouble Figuring out what this is. What does this do?
Well, it stays on top of the water surface purely based on surface tension, not buoyancy, like a water strider insect. It moves based on vibration.
The Vibration keeps it up? Wow,thats neat.
No, as I said, it stays on top of the water surface based on surface tension. It moves forward based on vibration (comparable to a bristle bot).
Ah,I got it now, I think...
thats pretty nifty! i saw those on the science channel a couple times. what size pager motor do you use?
I got it from www.bgmicro.com years ago. It is about 2cm total length. You can get smaller/lighter these days and the lighter, the better.
That is fascinating - I have never seen any mechanical device use surface tension in that way. This may be an utterly unique vehicle (at least, until people start following your lead!). Bravo.
I agree. It is quite unique. Also quite smart. The idea of using surface tension rather than buoyancy is amazing.
I agree its prototype and excellent paradigm of taking advantage 2 principles of simple Physics. 1. the Surface tense (iam Greek iam not sure the terminology) but it means the force of the water molecules and how an insect for example can walk on the surface just has more thin and expended legs on the surface like this instructable 2. The mechanical movement that is used by many plangton organisms. Vibration Here is electric in insects is biomechanical....but the principal of movement as i said is exact the same.... Bravo 5/5
<sub>It's surface <em>tension</em>.</sub><br/><br/><sub>According to <a rel="nofollow" href="http://online.ectaco.co.uk/main.jsp%3bjsessionid=bc30cd6a5bd33a391c7c?do=e-services-dictionaries-word_translate1&direction=1&status=translate&lang1=23&lang2=el&source=tension">this website</a>, it translates as &#941;&#957;&#964;&#945;&#963;&#951;, &#965;&#960;&#949;&#961;&#941;&#957;&#964;&#945;&#963;&#951; (I have no idea if that is correct!).</sub><br/>
thanx for the correction....:)))
Any time.
Doh, I should have read your link! OK, so it's not exactly unique, but it is probably far cheaper than the original STRIDE project beasties.
Here I am, late again. Cool bity little water strider though! Here is an idea for propulsion. Using the attached picture, notice the red line between the long arms? This line is very small dia. filament or thread and is looped over the motor counter weight via the shown brass guide. As the motor rotates, if the thread is properly tightened, the two long arms will pull slightly toward each other and then spring apart. Nearly no weight added and the pull and spring action would be close enough to horizontal that it should not cause a loss of surface tension... One idea anyhow, who know maybe I could be a closet genius...
Thanks for the suggestion. It will a.o. depend on the motor delivering enough torque, which is far from sure as it is ungeared. I will however have to wait for the pool season for more experiments.
If only i could hear what you were saying (if at all) it would make a beautiful science project.
I did not make much spoken comments and it was in Dutch anyway. All the explanation is written.
i'm doing this with my vibrobot
Very Cool !!! seems easy enough to make ...Have you made them larger or smaller ? If so did they work as well as the one shown?
Until now I only made one attempt at a smaller one. Keeping the weight even further down I tried a propulsion based upon a double nylon string under tension between two carbon legs. A small flat piece of plastic was put between the double string. Winding this builds up tension and letting it go this gives a "paddle boat type" propulsion. But the "padle" also gives a downwards push, making it sink.
Have you thought of a motor based on the bacterial flagellum? It uses a corkscrew action with fibrous strands to create forwards momentum and shouldn't have a problem with downwards force. One method I can see would be to have the motor above water and have the strand angled downwards, although that could lead to issues because of surface tension (ironically) and upwards momentum due to the angle. The alternative would be to use a straw or similarly bendable tubing to create a path the strands can go through from above water to get under water enough that they won't hit the surface and angle it so that the strands only cause a horizontal force.
interesting suggestion. Although I expect that a propeller would work better, staying within the "biological" theme makes it appealing.
If it floats with four legs at the back (I'll just call them legs) but the propulsion gives a downward force that sinks it, why not add another two 'legs' or even four, to create enough surface tension to stop it from sinking when it 'paddles'?
You can indeed add legs, but the paddle version was an attempt in making a smaller, even simpler version. Of course you can add more legs without making the overall size bigger. But mainly I concluded the paddle-type propulsion is not very suitable, at least not in its simple form. Meanwhile instead of trying to making it smaller, I'd rather wait for some good weather to test a BEAM-bot version at about the original size.
Please post a vid of it ....I would like to see it in action
another Idea I just had, what if you had another of those rods placed alongside the long one the main rod (the body-like piece) and put some plasticard between it, this would hugely increase surface area for only a small weight increase, not likely to sink then XD
I'm not sure if I understand correctly, but it is not the surface area that is important. It is the LINE-contact that determines the supporting force. Any surface that is already in contact with water does not play any part. It is at the edge between water and air that the upward force originates.

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