Cellulose sponge absorbing water

As liquid creeps up a cellulose sponge, microscopic holes in the sponge walls grow and merge. Credit: Jonghyun Ha and Ho-Young Kim, Seoul National University

Engineering

An absorbing study on the maths of sponges

Formula predicts speed at which dry material soaks up water.

A kitchen sponge wicks water upward by filling microscopic holes that quickly tear themselves apart.

Sponges are often made from plant cellulose, a porous material also used to manufacture paper. A team led by Ho-Young Kim at Seoul National University used a scanning electron microscope to observe the pores in a sponge as it took up liquid.

The researchers found that the pores’ walls are riddled with micrometre-scale holes that expand and coalesce on contact with water, effectively tearing bigger openings in the material. When the sponge dries, the micro-holes reform.

On the basis of these observations, the team derived a formula that predicts how quickly water rises in a sponge, including the slowing of absorption as the sponge becomes fully swollen. The new theory could be applied to absorptive materials in biomedical devices — and to bread, which also soaks up water at a rate described by the new formula, the team found.