Oops
I cannot quite explain (i) how a pencil makes a mark on paper and (ii) how does the eraser work.
Textbooks say the eraser is "stickier." Why would a graphite slab adhere to isoprene-isopbutylene rubber better than cellulose? If one checks the surface energy for hydroxylated and nonhydorxylated organic compounds on graphite, the hydroxylated ones have a higher binding energy or comparable one. I am not sure an eraser is stickier for pure graphite. Pencil lead has wax and clay fillers that might bind to the elastomers. In any case, the difference would be quite small.
One usually hears that graphite is easily flaked because the interaction between the lamellar layers is weak (in these Nobel days I've read that perhaps a hundred times). Weak as it may be, in vacuum graphite does not work as a lubricant at all
http://pubs.acs.org/doi/abs/10.1021/ed034p240
One needs to adsorb water at the surface for a thin (10 nm) flake to come out as a result of slight friction; this water is weakening interlamellar binding just below the surface. This actually makes sense: the interaction between the sheets is on the order of 20 meV per atom (of which 50% comes from dispersive interactions and 50% from electron correlation), while water interacts with the energy of about 100-150 meV/ring - the effect can be substantial. Perhaps paper might be weakening the interaction even better; the cellulose is hydrophylic itself and it absorbs moisture from the air. This wetting eases friction, so the pencil leaves a mark, but the same process makes the flake vulnerable to the eraser. It is not that the rubber sticks better to the graphite flake; rather, it has hydrophobic surface that does not have this weakening effect on the interaction between the layers at the subsurface. When the shear is applied on both sides of the flake, the flake exfoliates at the hydrophylic surface rather than the hydrophobic one. But that's just one of the possibilities.
How do pencils and erasers actually work?
Textbooks say the eraser is "stickier." Why would a graphite slab adhere to isoprene-isopbutylene rubber better than cellulose? If one checks the surface energy for hydroxylated and nonhydorxylated organic compounds on graphite, the hydroxylated ones have a higher binding energy or comparable one. I am not sure an eraser is stickier for pure graphite. Pencil lead has wax and clay fillers that might bind to the elastomers. In any case, the difference would be quite small.
One usually hears that graphite is easily flaked because the interaction between the lamellar layers is weak (in these Nobel days I've read that perhaps a hundred times). Weak as it may be, in vacuum graphite does not work as a lubricant at all
http://pubs.acs.org/doi/abs/10.1021/ed034p240
One needs to adsorb water at the surface for a thin (10 nm) flake to come out as a result of slight friction; this water is weakening interlamellar binding just below the surface. This actually makes sense: the interaction between the sheets is on the order of 20 meV per atom (of which 50% comes from dispersive interactions and 50% from electron correlation), while water interacts with the energy of about 100-150 meV/ring - the effect can be substantial. Perhaps paper might be weakening the interaction even better; the cellulose is hydrophylic itself and it absorbs moisture from the air. This wetting eases friction, so the pencil leaves a mark, but the same process makes the flake vulnerable to the eraser. It is not that the rubber sticks better to the graphite flake; rather, it has hydrophobic surface that does not have this weakening effect on the interaction between the layers at the subsurface. When the shear is applied on both sides of the flake, the flake exfoliates at the hydrophylic surface rather than the hydrophobic one. But that's just one of the possibilities.
How do pencils and erasers actually work?