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Why does a slightly damp sponge absorb water so much better than the same sponge when it is bone dry?

Chris Daniel
Colwyn Bay, Conwy, UK

Sponges for dish washing can be made of polyester, which has mechanical properties that make it useful as a cleaning material, as well as being soft to handle when dry or wet. However, the polymer is hydrophobic, or water-repellent, which means its surface energy is lower than the surface energy, or surface tension, of water.

A dry sponge will therefore tend to float when placed on water, only absorbing liquid gradually. It can be forcibly wet by being submerged, compressed and released so that water fills its open-cell structure. When the bulk of the water is squeezed out, there will be sufficient remaining water in the sponge for the internal surfaces to be almost perfectly hydrophilic, allowing water to enter and fill the sponge again.

When washing dishes, a detergent is usually added to reduce the surface tension of the water, making it more compatible with the low surface energy of a sponge, in addition to its primary purpose of helping to remove fats from surfaces.

Mark Dirnhuber
Bristol, UK

This phenomenon applies to cellulose sponges.

Cellulose is a polymer of glucose and owes its strength to extensive hydrogen bonding between numerous hydroxyl groups, holding the glucose chains tightly side by side. These also give it great affinity for water, so cellulose surfaces are very wettable because this attraction is even stronger than that between the water molecules themselves. This results in the wet cellulose sucking up the water into the pores by capillary action.

Dry cellulose will first need to be wet for this to happen, and there are energy barriers to be overcome, notably due to the surface tension of the water itself. Because the cellulose surface is rough, a water droplet might sit there for a while as a bead before the wetting takes place. Cellulose also absorbs some water into (not just onto) its structure, which will divert water from the surface and further slow the wetting.

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