Wrinkling behavior of variable thickness films bonded to elastic substrates

The wrinkling morphologies in film–substrate systems are formed spontaneously, which provides a promising way to introduce novel functions on material surfaces. However, wrinkling behavior of variable thickness films bonded to elastic substrates is still unclear, which restricts the blooming applications of tunable wetting, tunable optical and antifouling surfaces. In the present paper, critical strain and wrinkling morphology affected by various parameters for two typical cases of variable film–substrate systems, that is, films with thickness gradient and films with periodic surface morphologies, are studied first. Theoretical analysis is then conducted to uncover the mechanism of different wrinkling patterns. And the wrinkling behavior of some extended designs based on the two typical cases is finally explored. It is found that the critical loading and wrinkling morphologies for the two typical cases are quite different. Three buckling modes are found, that is, sinusoidal buckling, Euler buckling and rigid rotation for periodic surface topologies of different scale compared with the wrinkling wavelength, which agrees well with theoretical prediction. Wrinkling morphologies can be well tuned by adjusting the thickness difference, the lengths of thick and thin regions, as well as surface morphologies of local regions of the upper film. The findings should be useful for the design of film–substrate systems in engineering practices, and improve the knowledge of elastic instability.