Why is cling film so sticky when it touches itself or around bowls but it does not feel sticky at all when it touches our hands?
In: Physics
Materials can be classified according to how well they hold on to charge. Metals have free electrons that are shared among the atoms and can move with very little electric field. These are conductors. Other materials hold on to their electrons and require work (energy) to remove them. They will not move just because a weak electric field (force per charge) exists. These are insulators.
Among electric insulators, these materials can be further classified according to how well they hold on to their electrons. This is known as the triboelectric series. You can look this up and see which two materials from this series will hold on to electrons more. Rubbing one material against another will jostle electrons free. The material closer to the negative side (the better insulator) will end up with a net gain of electrons. These two materials will cling because of a further induced effect that will push on one charge and pull on the opposite charge. This series can explain why pvc is often used as insulating material for wires.
Your science teacher may have rubbed rubber with fur. These two are far apart on the series with the rubber gaining substantial net electrons. Paired with this demonstration is usually silk with a glass rod. But here, the silk is the one gaining the electrons from the glass. This effect is less pronounced than the first pair as silk and glass are much closer in their electron holding strength.
This is like rubbing a balloon on your hair and sticking the balloon to a wall. But here the wall was never rubbed and has no net charge. Uncharged insulators can stick to charged insulators when a charge polarization is induced.
The balloon netted more electrons from those jostled free by rubbing your hair. Your hair stands up because it has a net charge as well, but positive due to lack of electrons. The balloon has extra charge clumped near where they were pulled by rubbing on your hair and when brought near a wall (an insulator), it will push electrons in the wall away a little (reducing the repulsion) and then feel an attractive force because of this induced polarization (alignment of electrons further from the balloon than they were before).
So saran wrap or cling wrap is very far on the negative side of this series. So much so, that these materials often stick to themselves because of the effect mentioned above. Rubbing cling wrap very vigorously against itself may jostle some electrons free, but an equal amount will go to each side, so this is not from rubbing.
The effect works best with materials paired from the far opposite end of the triboelectric series. These still need to be insulators. Remember that a conductor under the influence of an electric field will simply react to the field by free electrons moving until the force on them is balanced.
So pair cling wrap with glass. Glass will hold on to its electrons, but they will respond more and produce that induced polarization to the greatest degree possible and thus produce the greatest clinging attractive force.
Your skin will not show up on most series lists, but you can expect that it would fall around wood, cotton, or perhaps hair or fur if dry. There are also far too many dissolved ions in your body that can react to this force and so it acts somewhat like a metal so there would be no effect.
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