User:Onceastaralwaysastar/sandbox/Wrinkles (materials science)

Wrinkles found in nature have intriguing material properties that have inspired recent advances in tuning the mechanical properties of 2D materials such as graphene.

Graphene
Graphene is of interest for its various applications to flexible and stretchable electronics, energy storage, and surface wetting properties. Graphene is inherently rigid and brittle with low bending tolerance. Intrinsic wrinkles observed in graphene negatively affect the mechanical properties of graphene. Intrinsic or spontaneously formed wrinkles are thought to occur due to thermal expansion, transferring of graphene between surfaces, and replication processes. Wrinkles introduce anisotropy, which is reflected in the directionality of crack propagation with respect to wrinkle structure under mechanical failure.

Recent work has explored methods to intentionally introduce wrinkles in graphene. Adding texture and pattern to materials can significantly alter material behavior and properties. Graphene wrinkles are desirable for increasing surface area, improving stress tolerance, manipulating optical transmittance and chemical reactivity, and modifying surface wetting properties. Three notable methods to create graphene wrinkles are 1) stretching of single-layer systems, 2) strain-relief of film-substrate bilayer systems, and 3) conformal graphene wrinkles in multilayer systems.

Stretching of Single-Layer systems
In Chen et al. 1D wrinkles were fabricated by coating shrink films with graphene at elevated temperatures and then releasing the strain in a controlled manner to achieve the desired wrinkle periodicity or wavelength. While the shrink film can only undergo one deformation, the graphene wrinkled layer can be separated from the deformed shrink film and transferred to a new shrink film to repeat the process for wrinkling in a different orientation or with a different wavelength. 2D wrinkles are also achievable with this method by releasing strain along two directions. However, 2D wrinkles resulted in disordered wrinkles or crumpled graphene in contrast to the ordered nature of 1D wrinkles. The single-layer system, however, cannot achieve dynamic and reversible modification of material properties via wrinkling. This is because after the graphene de-laminates or detaches from the substrate, stretching or compressing the substrate no longer affects free-standing graphene layer.

Film-substrate bilayer systems
Hu et al. addressed this issue of de-lamination by improving the interlayer adhesion in a bilayer system composed of PMMA and PDMS, two soft polymeric substrates. The improved interlayer surface adhesion was attributed to removal of liquid at the interface with a post-curing process, resulting in stronger Van der Waals bonding between the graphene and substrate.

Conformal Wrinkles in Multilayer Systems
Researchers have since advanced the engineering of graphene wrinkles, demonstrating the ability to pattern conformal graphene wrinkles in area-specific regions with differing wavelengths and orientations. Graphene wrinkles were fabricated by releasing strain on a pre-strained polymeric substrate with a graphene layer on top. Conformal graphene wrinkles were achieved by including a soft fluoropolymer skin layer for strain relief between the pre-strained polymeric substrate and graphene layer. This is a significant development, as graphene is much more brittle and likely to fail after de-lamination from the polymeric substrate. In fact, inclusion of the fluoropolymer skin layer was shown to increase the critical tensile strain at which cracking occurs by up to 670%. Mechanical modeling of the substrate skin-layer graphene system showed that plastic deformation of the soft fluoropolymer skin layer was key to achieving crack-free graphene wrinkles.