  Metamaterials enable the emergence of novel physical properties due to the
existence of an underlying sub-wavelength structure. Here, we use the Faraday
instability to shape the fluid-air interface with a regular pattern. This
pattern undergoes an oscillating secondary instability and exhibits spontaneous
vibrations that are analogous to transverse elastic waves. By locally forcing
these waves, we fully characterize their dispersion relation and show that a
Faraday pattern presents an effective shear elasticity. We propose a physical
mechanism combining surface tension with the Faraday structured interface that
quantitatively predicts the elastic wave phase speed, revealing that the liquid
interface behaves as an elastic metamaterial.
