Meniscus propulsion

wywong

Earlier this year, a team of scientists demonstrated a tiny boat that uses surface tension, but no moving parts to navigate through water (see http://www.pitt.edu/news2009/Cho.pdf or http://www.scientificamerican.com/ar...walks-the-walk).

I wonder if a boat with hydrophilic material at the back and hydrophobic material at the front can propel itself in the same manner. If not, why? If so, does that mean a perpetual motion machine can be made using a circular trough and one such boat?

Wai Wong

wywong

After thinking through, I figured out (I think) why a meniscus can't propel a boat. The meniscus itself is stationary, so the horizontal forces acting on it, which include surface tension as well as water/air pressure, must be balanced. There is only one horizontal force - water surface tension - acting on the far side of the meniscus. That shows that the forces - surface tension and water/air pressure - acting on the near side of the meniscus must add up to be the same. By Newton's 3rd law of motion, the side of the boat must experience the same force whether there is meniscus or not. So the boat won't move unless the water surface tension on either side is different, as in the case of those toy boats that use chemicals to change surface tension.

I think the experimenters' boat movement was caused by the *externally* applied electric field. Had they used on-board batteries, the boat wouldn't have been able to move.

Wai Wong

cesiumfrog

Is that conclusion consistent with your source's example of some larvae's locomotion?

wywong

Another team of scientists led by D.L. Hu had earlier invented a robotic water-walking insect with moving parts and their work is the only reference in S.K. Cho's paper. According to Hu's observation, those insects move by generating vortices around their legs. It is plausible that the larvae's locomotion is based on the same principle. I know almost nothing about insects so I have no way to tell for sure, but Cho's interpretation seems implausible to me.

Wai Wong

cesiumfrog

I get your point about the thermodynamics not being explained, but isn't it premature to make such a conclusion about all manners (incl. mechanical and chemical) of surface-tension propulsion, before finishing your alternate explanation for Cho's apparatus?

wywong

Sorry for any misunderstanding.

In Cho's analysis, he only took into account the surface tension forces and overlooked the forces acting on the walls of the boat due to water and air pressure. In my argument, if those forces are considered, the meniscus can be ignored - the horizontal force acting on the wall is the same whether there is meniscus or not.

When I said meniscus propulsion is impossible, I meant Cho's unique meniscus propulsion, because when I googled the term 'meniscus propulsion', all the matches I got were related to his "discovery". If a propulsion is caused by any other manner, I suppose some other name should apply, like mechanical propulsion, surface tension propulsion, etc., because the meniscus doesn't affect the total horizontal force.

Wai Wong

cesiumfrog

I wonder if the better analogy to Cho's boat is the operation of soap boats (noting since an oscillating potential is in use we can't assume power isn't being continuously expended into the water), rather than the the larvae behaviour Hu observed (as Hu was specifically describing "meniscus climbing" edge-effects not locomotion in mid-pond, although admittedly the surface won't be exactly flat anywhere). Nonetheless, WW, I'm eagerly looking forward to seeing the precise details of your calculations, both of the exact horizontal cancellation of surface tension by pressure force, and of the electromagnetic explanation for the demonstrated manoeuvring.

wywong

Cesiumfrog you are right. I made a big mistake in my previous argument. I previously argued that the surface tension force of a meniscus is balanced by the static water pressure so that the perpetual motion boat is impossible. I think that argument is correct as far as the perpetual motion boat is concerned. Cho's boat is different in that the meniscus at the back is moving up and down. So I did the following experiment:

A loaded triangular paper boat is put in the middle of a basin of water. A pencil parallel to and near the back of the boat is pushed into and out of water at a rate of about twice a second. The boat is observed to move forward until it hits the wall in a few seconds.

Surface tension force does not seem responsible for the propulsion of the paper boat above. My explanation is as follows. When water is pushed downwards quickly, it is pushed against surrounding water which is quite heavy and viscous, so the water pressure nearby increases considerably, and it is this pressure that propels the boat. When water is drawn upward, there is negative pressure, but that pressure is limited - the pressure at the back of the boat cannot be lower than the atmospheric pressure by any significant amount, while the pressure built up at the front is small because of the boat's slow speed. As a result, the boat gets a big push and small pull in each cycle. This propulsion mechanism can also account for the larvae's locomotion.

Thus, while Cho's invention is sound, his explanation is dubious, and I proved myself a fool B:(.

Wai Wong

sophiecentaur

Soap and camphor propelled boats actually 'consume' some of the substance which coats the stern / engine as they move. The energy for the propulsion comes from the altered molecular arrangement as the camphor / soap spreads out into the water. No conservation law is violated. The difference in meniscus angles at front and back is only maintained by a 'chemical' source of energy. The boat stops when the propellant has all gone away.