Polymer thin films

Polymer based conductive and transparent thin films are an important class of functional materials at the heart of flexible organic electronic devices. Stacking printed thin films layer by layer is a low cost and frequently used technique to build structured devices with the desired electronic properties and functionalities. The mechanical stability and adhesion between the adjacent layers is key in the performance and lifetime of such devices. Thin flexible films are, however, prone to a number of instabilities: cracks, tears, or delamination and wrinkling. This is detrimental to their use especially in the case of multilayer devices. 

Here, it is shown that a simple water or solvent drop deposited on such films gives rise to a buckling instability and the formation of several folds due to the tendency of these films to swell in contact with the solvent. We found that the number of wrinkles at the front of drop spreading increases linearly with the drop size. Depending on the properties of the films such as thickness and Young’s modulus, the films could either exhibit wrinkling or stable exposed to the solvent drop. By using an energy balance (bending, adhesion, and interfacial energies due to wetting), we proposed a model to describe the wrinkling instability (see the Figure).