Liquids flow downhill right? Wrong! Under the right circumstances they can sometimes go uphill too.
The clip below shows how, if you vibrate a surface violently enough, between around 30 to 200 times a second, and use a liquid that is a little thicker than water, droplets can be made to climb uphill.
Mathmatician Jens Eggers from Bristol University explains it like this:
"As the shaking plate rises the drop is compressed, while it bulges upward as the plate falls. If the shaking is vigorous enough to overcome the surface tension experienced as the drop is compressed, the drop will tend to lean forward, producing a net force which drives the drop uphill."
It's a neat trick, and one that could be useful for manipulating small amounts of fluid inside devices used in chemistry, biology and medicine. Such applications might not be relevant to all of us, but I think anyone can appreciate the video.
Well this post seems quite absurd because the guy is actually vibrating the surface which means that a force is being applied. Therefore it is quite obvious that the liquid would flow uphill. well anything would "defy gravity" by the help of a force. When we jump; we "defy gravity" , but as soon as the force is cancelled, gravity catches up. So why dont this guy stop vibrating the surface and see what happens. Would the fluid still move uphill? Therefore the point remains that u apply force,: u could move anything uphill.
(I think) Some of the f has been used to move the water molecules, becuase those molecues have inertia gravity will act on these molecules but after the plate has returned to its initial position. which is why the blob bobs around. This is nothing new, next time it's wet and you've polished your car watch the water blobs on the bonet.
>But the force is cancelled here. The >plate moves the same distance up and >down with every 'cycle' of vibration.
>If you'd asked me, I'd have said >that would return things to their >original state. But this show that >more complex effects are at work.
Sorry, you're wrong.
First off all the upward stroke is obviously stronger than the effects of gravity (if it wasn't it wouldn't be moving upward).
Secondly, it is this "stronger-than-gravity" upward force that is pushing the droplet up. The only forces coaxing the droplet back down are gravity plus an insignificant pulling effect caused by the liquids surface tension "sticking" it to the ramp.
So in summary: Upward force > gravity Downward force = gravity
Anon>But the force is cancelled here. The plate moves the same distance up and down with every 'cycle' of vibration.
Yes, and in the meantime the drop has moved a little bit.
Anon>If you'd asked me, I'd have said that would return things to their original state. But this show that more complex effects are at work.
You have not considered the time between the plate moving up and the plate moving down. Read the article. The drop is compressed and before it has a cance to spring back the plate gives it another shove up the 'hill'. Thewre are no complex effects here. There is nothing mysterious going on. No pseudoscientific nonsense required.
If you read the article carefully, you will find that the droplet is not being shoved uphill by the vibrations, nor is the upwards force greater than the downwards force. If the is the case then we would also see the slope moving upwards as it vibrated. Instead the shaking is causing the droplet to lean towards the slope, thus falling into the slope, migrating upwards. The is because a vertical force is being exerted on a sloped surface, and the droplet is being compressed and stretched off it's center point. Not psuedoscience, merely mathematics
I would hazard an intuitive guess that the frequency of the vibration matters and that different liquids would exhibit this behaviour at different frequencies, perhaps a "resonant frequency" based on the viscosity and surface tension of the liquid ?
The impressive thing is not about the force or the source of energy, but I believe that you would find it hard to move the droplet upward using only your bare hands(unless you find a good liquid for it). It is the frequency that is hard to find!