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Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
This is a demonstration of an article from New Scientist Jan 21 2006 (p14). In essence water (or other liquid) droplets placed on a heated surface (well above boiling point) will roll around. If you put them on a ratchet surface drops can be made to run uphill by about 10 degrees.
Here is my setup: Photo 1 A pile of transformer laminations on an angle provide the ratchet. A drop of water from the tube drops then starts to rise up the hill towards the left. It happen rapidly. There is a spirit level in the background and the photo is level. Photo 2 shows several drops Photo 3 shows heating method with a MAPP gas blowtorch. A range of about 200 - 300C seems best but there does seem to be a sweet spot temp where it works best. The article used 220C but smaller drops and smaller rachet . Photo 4 shows that if it is too cool eg 140C then the drop wets the hot surface and explodes.
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
water drops are suspensed by a vapor layer, they just kinda bounce around under normal condition, but the ratchet action makes them go in one direction.
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Andrew wrote ...
the mechanical bit I get. what is not clear to me is how the potential energy gained by the drop is extracted from the thermal energy of the surface.
There is a huge flow of energy from the hot surface to vaporize the water. It only takes a little assymetry to give a sizable force in a given direction. How this happens is not exactly clear and the authors of the article were also a bit vague on this. Surface tension must play a big part but I can't really work it out to my satisfaction. I had taken about 300 photos but no videos as yet. Some interesting points. With some drops, possibly large ones, there is an optimum drop distance to best acceleration. A lot of the initial push comes from the first hit and often the drop would shoot up the hill, slow, then roll down again. There are lots of variables here which include drop size, height of fall, temperature of the ratchet, ratchet, slope, spacing and height. Optimum for me seems to be as demonstrated. My slope was about 9 degrees. The writers used 10 degrees and used 1mm drops and 0.1 mm ratchet. Speed of their drops was 5cm/sec and they could travel up to 1 meter before evaporating. Photo 1 shows a drop travelling fast enough to become airborne over the top of the ski ramp. Photo 2 shows drops of different shapes. Probably partly due to size but also due to the forces that agitate them a lot as the specular reflections are no longer clear.
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Water drops walking uphill
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