  Graphene is only one atom thick and becomes the ultimate thin film to explore
membrane physics and mechanics. Here we study hierarchy of graphene wrinkles
induced by thermal strain engineering and demonstrate that the wrinkling
hierarchy can be accounted for by the wrinklon theory. We derive an equation
{\lambda} = (ky)0.5 explaining evolution of wrinkling wavelength {\lambda} with
the distance to the edge y observed in our experiment by considering both
bending energy and stretching energy of the graphene flakes. The prefactor k in
the equation is determined to be about 55 nm, which is independent of the size
of the graphene flakes. Our experimental result indicates that the classical
membrane behavior of graphene persists down to about 100 nm of the wrinkling
wavelength.
