Soap Powered Propulsion
It starts with a primary school science experiment
Someone showed me a science experiment I had never seen before. The setup is: you have a bowl with water in it and you grind some pepper onto the surface. Watch what happens when you add a little bit of soap onto the surface of the water:
What causes this to happen?
When soap is introduced onto the surface of water, it will quickly start spreading over the surface. The macro view of why this happens is that the soap film reduces the surface tension of the water and puts it into a lower energy state. Things prefer to be at a lower energy state the same way a ball at the top of a hill will roll downhill.
The micro view is that soap molecules have both a hydrophilic end and a hydrophobic end. The way this interacts with the water is what causes the soap film to spread on the water surface.
Here’s a more detailed explanation for anyone interested:
Can this be used to move objects?
After looking at the initial experiment, I wondered if this could be used to propel an object forward. As it turns out, it can:
But this still feels like cheating because the soap propels the object forward and I put it there using my hand. The easy solution is to add soap onto the "boat" and use a wick to release the soap into the water slowly.
Controlling the motion
This is getting better, but there's no way to control when the boat moves forward.
It would be really cool if we had electronics that dipped soap onto the water only when we wanted it to move. First though, would this even move if it had the weight of electronics on it?
The test above shows that 15gms is a reasonable weight limit for a 8cm x 8cm square of foam.
Next we need to whip up some electronics.
To communicate with the micro-controller I would normally pick Bluetooth, Wifi, or USB. But because of the weight limit a WiFi module would be too heavy. Instead I went with a light sensor. I shine a laser onto the light sensor when I want the boat to move. I've shown an example video of an older prototype below. This one doesn't have soap and so doesn't move.
For the actuator I used a piezo-vibrator because it was the lightest moving thing I could find. Because vibrators aren't really designed for this use case, I had to try a bunch of different actuator combinations for this to work. The final set-up looked like this (the boat doesn't move because no soap is used in this demo):
It's a vibrator on a piece of paper that acts like a lever and also has a thin metal rod attached to it. The vibrator moving up and down moves the paper, which amplifies the motion and also moves the metal rod. I dab a bit of soap on to the metal road so that the soap only touches the water when the vibrator is on.
Final Result
Here's the final result in action!
It would be really cool if we could also stop the boat; but once the soap touches the water it continues getting drawn in to the water. We could likely solve this issue with a vibration setup that has a larger range of motion. Another issue is that the soap runs out in the demo above. I would probably connect a larger soap source to the actuator through some tubes or a wick.
For the moment, I'm quite happy with where this got. In the future if I have the time to find or make a better actuator I might test it out in a similar system to try and improve it.
Happy accident
For now, here's another prototype that had a really interesting outcome. The actuator had a flap that acted like oars on a boat and this let the boat move forward without any soap involved.
Here it is in action below.