This has been bothering me for a… well… for a while now.
I have several synthetic sponge balls (kids play balls, etc) that have all kinds of hairs weaving in and out of them. I can’t pull the hairs out without breaking them.
How do hairs weave in and out of synthetic sponges like that?
I guess a related question is:
How are synthetic sponges made, anyhow? I always figured they were made of some sort of plastic or rubber foam.
Does anyone know how to rid the sponges of the hair?
Thanks
Hair is funny stuff; take a long strand of hair at one end and pull the length of it through your pinched fingers; it feels smooth; try the same thing from the other end and it sticks or feels rough; this is because the surface of the hair is composed of a multitude of tiny overlapping scales, like the shingles on a roof.
How does this make the hair work its way into the sponge? well, those overlapping scales work like the barbs of a harpoon; they go in easier than they come out, so any movement (usu. squeezing the sponge) that causes the hair to try to move against the material of the sponge will be much more effective in one direction that the other and thus the hair works its way in.
In exactly the same way it is possible for hairs to work their way into the pores in your skin; notably that on the sole of your foot - this happens to me once in a while and it hurts a great deal.
Capill-hairy action?
I can buy that general theory. What I can’t figure out is why your feet remain in contact with any one particular hair long enough for it to work its way into the skin. Are you a hobbit, or something? Even my paws aren’t that hairy. 
I will assume that your sponge balls did not come out of their packages with embedded hairs and that the hairs your are describing are the typical human/dog/cat/etc. hairs that one finds around one’s house. If either of these assumptions is incorrect, you’ll need to keep looking for an answer.
Mammalian hair generally has a layer of scales on it. On many animals, these scales tend to “face” one direction. Sponges (whether natural or manufactured) are porous–they not only have the holes that you can see, but their surfaces also have many smaller holes that are nearly invisible.
A hair landing on a sponge (or sitting on the floor and being fallen upon by a sponge) has some non-zero chance to have one end pierce the sponge’s surface through one of the tiny holes. At that point, the scales begin to act as the teeth of a ratchet, allowing the hair to move into the sponge when the surface is compressed, but preventing it from withdrawing . Once inside the sponge, the end of the hair may be deflected by the sides of the larger holes that make up that interior, so that the hair may begin curling about, further anchoring itself to the sponge.
Eventually, the hair is firmly caught within the sponge at one end, while the remainder of it may not have yet been “swallowed” by the sponge.
And, on preview, I see that Mangetout already answered, but since I’ve already typed this, I’m going to post it, anyway.
It happened a lot more in my adolescent years, when I lived in a house where there was a dog and spent a lot more time walking about witout footwear; remember, it’s like a harpoon; once the freak event of [pore happening to land directly on the right(actually ‘wrong’) upturned end of a hair], then it’s all downhill from there. Not terribly common (probably happened to me a dozen or so times in my life), but painful enough to stick in the memory.
Hair stylists I know call these hair slivers. According to them t’s fairly common in that trade.
Huh. It never would have occured to me that that was possible. I guess that’s one good thing about never being able to have a dog when I was young. Thanks! 
