Pattern Transformation

Mechanical instabilities in soft materials are often considered as failure mechanisms in device performance. Recently, researchers have discovered that mechanic instability can trigger dramatic pattern transformation in periodic structures. These instabilities can be harnessed to assemble complex patterns for various applications in tunable photonic and phononic crystals, structural colors, and negative Poisson ratio materials.

1. Fabricated 2D photonic crystal slabs with anisotropic unit cells using pattern transformation

In comparison to the three-dimensional (3D) PCs, 2D PC slabs offer efficient alternatives that can retain or approximate many of the desirable properties of 3D PCs, and be mass producible over a large area with submicron resolution. The most widely studied 2D PCs are square and triangular lattices with isotropic circular-shaped unit cells. In this study, we developed an efficient method to fabricate a rich library of photonic crystal structures with anisotropic unit cells via harnessing the pattern transformation of a poly(dimethylsiloxane) (PDMS) membrane. We also studied the photonic bandgap (PBG) properties of the resulting 2D photonic structures as a function of their structural symmetries and their tolerance to the structural deviation.

Further Readings:

Zhu, X.L., Zhang, Y., Chandra, D., Cheng, S.-C., Kikkawa, J. M., and Yang, S., “Two-dimensional photonic crystals with anisotropic unit cells imprinted from poly(dimethylsiloxane) membranes under elastic deformation”, Appl. Phys. Lett., 2008, 93, 161911. [DOI]

2. Revealed cappilarity induced instability in responsive hydrogel membranes with periodic hole array

We reported capillary force induced instability from drying the water swollen poly(2-hydroxyethyl methacrylate) (PHEMA) based hydrogel membranes with micron-sized holes in a square array. When the PHEMA membrane was exposed to deionized-water, the size of the holes became smaller but retained the shape, so called breathing mode instability. However, during the drying process, the square pore array buckled into a diamond plate pattern. The deformed pattern could be recovered upon re-exposure to water. The deformation and recovery cycle could be repeated many times. When thermoresponsive poly(N-isopropylacrylamide) was introduced to PHEMA gel, the poly(2-hydroxyethyl methacrylate-co-N-isopropylacrylamide) (PHEMA-co-PNIPAAm) membrane underwent pattern transformation only if dried below the lower critical solution temperature of PNIPAAm. Along the pattern transformation, we observed dramatic change of the optical property of the film, from colourful reflection to transparent window.

Further Readings:

Zhu, X. L., Wu, G. X., Dong, R., and Yang, S., “Capillarity induced instability in responsive hydrogel membranes with periodic hole array”, Soft Matter, 2012, 8, 8088-8093. [DOI]