Wetting of Auxetic Metamaterials

It is a common conception that when a material is stretched it becomes thinner. However, when an auxetic metamaterial is stretched it becomes wider. This is because the properties of an auxetic material are determined by its lattice arrangement and not by the materials properties of the individual solid elements. Because the expansion of the solid surface area is due to increased space between the solid elements, this allows new types of superhydrophobic, hemi-wicking and liquid-infused surfaces. To illustrate these ideas, we describe the theory for a hydrophobic bow-tie element based lattice constructed with joints that rotate under strain to create a hexagonal lattice. This creates an auxetic superhydrophobic material with a negative Poisson's ratio which can be converted by strain to a superhydrophobic material with a positive Poisson's ratio. We illustrate these ideas experimentally using surfaces designed with micro-scale bow-tie lattice structures. We show how the wetting and superhydrophobicity of these surfaces behave under strain for both positive and negative Poisson's ratio.