Why is a sponge easier to bend when it's wet?

A cellulose sponge  (and even a plastic sponge that doesn't seem to change size when it's wet) is much more flexible when it's wet.  What's happening?

Comments

[canucksgirl]

Whether a sponge is natural or synthetic, it is built up of cells that form holes which allow it to absorb water.

When the sponge is dry the cells contract making it fairly rigid especially if its a natural sea sponge. It may not appear to change size much at all, but it in fact does throughout its entire structure.

When water is introduced, the cells expand and it becomes more flexible.

Its like a piece of tall grass. When its 'wet' in its natural state, it's fairly flexible; but if you pull it out of the ground and let it dry, it will likely snap if you try and bend it.

[mole1]

So in the swelling (by adding liquid), it's the elongation of the walls between points of contact that makes them buckle more easily?

[canucksgirl]

Well I'm not a Scientist, but from what I recall, the structure of a sponge (when dry) contracts creating a tighter more rigid shape. When wet, the structure (as you said) elongates making it more flexible.

[CameronSS]

Ever played with a "push puppet" where pressing a button on the bottom loosens the strings, and it collapses? Same sort of thing. As it dries up, the strings tighten, and everything is pulled together. Wetting it loosens the strings, and everything goes wobbly.

[canucksgirl]

Good analogy... i was looking for a similar explanation, but it wasn't coming to me at the time.. Good call.

[Re-design]

The water lubricates the joints between the cells and makes the sponge more flexible.