Effect of lateral dimension on the surface wrinkling of a thin film on compliant substrate induced by differential growth/swelling

• The role played by the film's lateral dimension in its surface wrinkling revealed.
• The basic phenomenon was established through experiments.
• Finite element simulations were performed to understand the buckling mechanics.
• Theoretical analysis led to closed-form predictions for the critical growth strain.

Surface wrinkling in thin films on compliant substrates is of considerable interest for applications involving surface patterning, smart adhesion, liquid/cell shaping, particle assembly, design of flexible electronic devices, as well as mechanical characterization of thin film systems. When the in-plane size of the system is infinite, the critical wrinkling strain is known to be governed by the moduli ratio between the film and substrate. Here we show a surprising result that the lateral dimension of the film can play a critical role in the occurrence of surface wrinkling. The basic phenomenon was established through selective UV/Ozone (UVO) exposure of a strain-free PDMS slab via composite copper grids with different meshes, followed by treatment using mixed ethanol/glycerol solvents with different volume fractions of ethanol. To understand the physics behind the experimental observations, finite element (FE) simulations were performed to establish an analytical expression for the distribution of shear tractions at the film–substrate interface. Subsequent theoretical analysis leads to closed-form predictions for the critical growth/swelling strain for the onset of wrinkling. Our analysis reveals that the occurrence of surface wrinkling and post-wrinkling pattern evolution can be controlled by tuning the lateral size of the thin film for a given moduli ratio. These results may find broad applications in preventing surface wrinkling, creating desired surface patterns, evaluating the interfacial shear strength of a film/substrate system and designing flexible electronic devices.

For a thin film resting on a soft substrate, the critical condition for the onset of surface wrinkling and the wrinkling pattern evolution depend not only on the moduli ratio but also on the lateral dimension of the film.Figure optionsDownload as PowerPoint slide