Sensing Changes

Superhydrophobicity comes from the amplification of surface chemistry effects by the small-scale roughness of the surface. One example of this is a that a hydrophobic surface can become superhydrophobic when it is made into a foam. However, if it is possible to switch off the water-fearing surface chemistry, the foam will become a super-sponge and absorb water rather than repel it.

A droplet on top of a foam block
A water drop rests on top of a superhydrophobic surface (with a high contact angle).
Droplet being absorbed into a sponge
When superhydrophobicity is turned off, the droplet is absorbed into the sponge.

In these experiments the hydrophobic surface chemistry is switched off by heating the foam above a designed-in trigger temperature. The first image shows a droplet of water (with a pink food dye) on a superhydrophobic foam. The second image is the same foam once it has been heated above the designed trigger temperature. The foam itself looked no different the droplet of water was slurped up. This material could be used as a simple indicator of environmental history. For example, (in principle) it could be used to provide an alert that a food sample has exceeded a defined temperature at some point during its past.

The switching of superhydrophobicity can be designed to be triggered by many physical effects and not just heat exposure.

Further Information

Read more about these experiments in the publication list below. This effect is described by Nature as a super-sponge with a super-slurp.