Walking on water: start building a StriderBot

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 are inspired by water strider insects.
I wanted to try out just how difficult it is to make a robot that moves on top of the water surface purely based on surface tension. Well, I found out that recreating the principle is not hard at all. You need a material that is light and stiff, at least somewhat hydrophobic and available in shapes giving a lot of "contact edge". At the end of the video I show how the construction sinks when disturbed, proving it is really the surface tension that keeps it on top of the water.

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 and basic construction

A simple test showed that the small diameter carbon fiber reinforced rods you find at modeling shops stocking indoor flying materials work fine. I used 0.8mm diameter rods, but the more commonly available 1 mm diameter probably are worth a try.
With two rods each about 1 m length of I made a construction as shown in the video, supporting a pager motor and a button cell battery. The rods are bent and kept into place by attaching them to each other wit ordinary cellotape. At the center "node" I made support with some more cellotape, keeping motor and battery out of the water. The motor was fixed with some double side tape, making sure the excenter weight turns freely. The wires coming from the motor had their electric isolation removed over a couple of mm and were attached to the battery with tape. This method was inspired by the "Evil Mad Scientist BristleBot".

Comments

[Captain Molo]

Sweet ive gotta try this!

[Random user]

Rather than using cellotape to secure the joints, can I use solder instead?

[masynmachien]

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.

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.

[Random user]

Thanks for the fast reply, helps a lot. Haven't actually started this project though, but hoping to soon :)

[imrobot]

amazing!!! however, does the motor short out after it sinks?

[masynmachien]

No it doesn't, some current leaks through the water, but that is far from shorting out 

for mor info on low voltage motors and water, see:  http://www.instructables.com/id/Simple-solar-powered-pocket-submarine-with-depth-c/

[imrobot]

ahhh... thank you!

[macrumpton]

I wonder if having the motor hooked up to a small air propeller might be more propulsive without disturbing the water and sinking it?

[masynmachien]

It might be tricky, but it could work.
The motor would need to be mounted on some pillar and a problem might be the pillar acting as a lever and the propulsion force pushing the front of the construction down.

[fwjs28]

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

[masynmachien]

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.

[astrong0]

can thin silver metal wire work? its 26 gauge wire

[masynmachien]

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.

[astrong0]

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?

[masynmachien]

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.

[astrong0]

dang...how bout fibre glass strips impregnated with resin then formed to the shape

[masynmachien]

That would probably work. But if you go into that effort it would be better to use carbon fibre.

[hammer9876]

Brilliant!

[ReCreate]

Uhm...I am Having Trouble Figuring out what this is. What does this do?

[masynmachien]

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.

[ReCreate]

The Vibration keeps it up? Wow,thats neat.

[masynmachien]

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).

[ReCreate]

Ah,I got it now, I think...

[Sergant Tinkers]

awesome!

[shiboohi]

thats pretty nifty! i saw those on the science channel a couple times. what size pager motor do you use?

[masynmachien]

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.

[Kiteman]

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.

[LEEROYJenkins]

I agree. It is quite unique. Also quite smart. The idea of using surface tension rather than buoyancy is amazing.

[agis68]

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

[Kiteman]

<sub>It's surface tension.

According to this website, it translates as ­νταση, υπερ­νταση (I have no idea if that is correct!).</sub>

[agis68]

thanx for the correction....:)))

[Kiteman]

Any time.

[Kiteman]

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.

[BIGBUG]

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...

[masynmachien]

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.

[lycoris3]

If only i could hear what you were saying (if at all) it would make a beautiful science project.

[masynmachien]

I did not make much spoken comments and it was in Dutch anyway. All the explanation is written.

[bob the builder #1]

i'm doing this with my vibrobot

[thewoodcarver]

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?

[masynmachien]

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.