Nature's Raincoats

Superhydrophobic Metamaterials

Our everyday experience is that if we stretch a material like a rubber band, it gets thinner in the transverse direction (until it breaks). Similarly, if we squash a material it gets thicker in the transversedirection. These cases, illustrated schematically below, are said to have a positive Poisson’s ratio.

A mechanical (auxetic) metamaterial is counterintuitive and expands laterally as it is stretched longitudinally and contracts laterally when compressed longitudinally, as illustrated schematixally below.

These materials have a negative Poisson’s ratio. An example of an auxetic material is an auxetic foam:

Auxetic materials expand laterally because of the arrangement of their lattice elements, which can themselves be inextensible, e.g. by rotation about their joints. Examples of this involving square and triangular lattice elements are shown schematically in the image below.

Auxetic materials have a range of current and potential uses. For example, Nike’s Free running shoes have auxetic soles because they cushion impact and can be shaped into double-curved surfaces.

Sole of a Nike women's flykit running shoe,
Sole of a Nike women’s flyknit running shoe

For more information on auxetic metamaterials, including further examples of their uses, see Shruti Mandhani’s Vitae 3-minute thesis talk on YouTube here and below:

If the surface of an auxetic metamaterial expands its surface area, but none of the solid elements expand, all of the expansion goes into the space between them. From a wetting perspective, empty space between solid elements is completely hydrophobic. Therefore, when an auxetic metamaterial expands, there is a larger proportion of hydrophobic areas in the total surface area. This means that straining a hydrophobic auxetic metamaterial can transform its surface into a superhydrophobic surface. The contact angle between a water droplet and a suface gives a measure of how hydrophobic the surface is, with a higher contact angle indicating higher hydrophobicity (as we discussed here). In the images below, we can see that the highest contact angle and hence highest hydrophobicity (middle panel) occurs between auxetic and conventional configurations.

Droplets on auxetic and conventional materials

You can read more about this process in the papers below.

Publications

Back – Research involving superhydrophobicity