Transforming Auxetic Metamaterials into Superhydrophobic Surfaces

Superhydrophobic materials are often inspired by nature, whereas metamaterials are engineered to have properties not usually occurring naturally. In both, the key to their unique properties is structure. Here, it is shown that a negative Poisson's ratio (auxetic) mechanical metamaterial can transform into a unique superhydrophobic material. When stretched, its surface has the counterintuitive property that it also expands in the orthogonal lateral direction. The change in the solid surface fraction as strain is applied is modeled, and it is shown that it decreases as the space between solid elements of the auxetic lattice expands. This results in a unique dependence of the superhydrophobicity on strain. Experimental models are constructed to illustrate the relationship between different states of strain and superhydrophobicity as the lattice transitions from an auxetic to a conventional structure. The findings offer a new approach to designing superhydrophobic materials for self-cleaning surfaces, droplet transportation, droplet encapsulation, and oil–water separation.