Wrinkling instabilities in polymer films and their applications

Surface wrinkles are commonly observed in polymer films driven by mechanical instability when the stress exceeds a critical value. Recently, wrinkling instability has been utilized as a versatile patterning platform to create unique surface patterns for a wide range of applications that are related to surface topography and its dynamic tuning. In this review, we discuss three mechanisms to create large‐area surface wrinkles via thermal stress, osmotic pressure and mechanical stress applied on bilayer and gradient polymer films. We briefly compare the governing physics in each system, and how to control the wrinkle pattern order, characteristic wavelength and amplitude, orientation and interactions under different geometrical confinements. We then present various technological applications that harness wrinkling effects, including optical components, responsive microfluidic channels, thin‐film metrology, tunable wetting and directed assembly of liquid crystal molecules, flexible electronics and particle sorting. We review thermal stress‐, osmotic pressure‐ and mechanical stress‐induced surface wrinkling instabilities in polymer films and how to control the wrinkle size, order and orientation. For each system, we present various technological applications that harness wrinkling effects.