A small insect can "walk" above the water surface because of surface tension, Y (see image). Now there is this problem in my physics textbook: A pencil 1.0 cm in diameter is held vertically in a glass of water. The water wets the pencil so the contact angle is 0<sup>o</sup>. Calculate the magnitude and direction of the net force on the pencil due to surface tension. I calculated the magnitude correctly, but while I thought the direction is upward (like the insect leg case), the book said that the direction is downward! Anyone care to give me an explanation???
I am <b>guessing</b> here so don't take my word. You have been warned. Please Register or Log in to view the hidden image! The insect floats because its foot remains dry and doesn't break the surface tension. The pencil sinks because it is wet so has broken the surface tension. A simple experiment would be to use a soapy soloution and see if the insect still floated with a reduced/non-existant surface tension. Maybe food for thought.
> The pencil sinks because it is wet so has broken the surface tension. Yep, that's what the book says (direction of force is down) Please Register or Log in to view the hidden image!, but I still can't reason why... Btw for the soapy solution, the surface tension would be very very small. (and can only support a lot lesser weight).
Wetted in, unwetted out If the pencil is wetted then going into the water is exothermic and driven by negative free energy change. If the pencil were hydrophobic and not wetted, it would be excluded for the same reasons but in the opposite direction. Cf: capillary wetting and capillary exclusion, respectively.
I don't understand. If the pencil is already wet, how would surface tension affect it? The water molecules are already sticking randomly to the surface of the pencil instead of being aligned with each other’s dipoles, so it wouldn't seem that any surface tension would exist on/around the pencil. Am I missing something?
Water is attracted to the pencil. Now at 0 degree angle, there is no water above the surface so no pull up. But at the surface and for a small distance below the pencil has a net attraction by the water. The key words here are "for a small distance below". This means that the bit of the pencil at the surface is pulled down by the water just below the surface. (via the water in between that is pulling either end.) Ignoring the floating presure stuff... further down ... water a bit lower is still pulling down, but water a little up is pulling up cancelling the effects. For the spider. Water is attracted to water and water is repelled by the foot. Strictly speaking not reppelled but the pull to "other" water is stronger than the pull to foot giving the same net effect In the dimple the water is pulled up by the sides of the dimple, and pushes up the foot in the middle/bottom of the dimple. Clear ???
Ah! Yes, that makes sense. I wasn't thinking of the pencil as resting on the surface of the water. Thanks.
