Capillarity induced instability in responsive hydrogel membranes with periodic hole array

We report capillary force induced instability from drying 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 instability mechanism was confirmed by comparing the observations from optical and scanning electron microscopy (SEM) images with theoretical prediction. When thermoresponsive poly(N-isopropylacrylamide) was introduced to the 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 a dramatic change of the optical property of the film, from colourful reflection to transparent window.