Guided wrinkling in swollen, pre-patterned photoresist thin films with a crosslinking gradient† Chi-Mon Chen, Jason C Reed and Shu Yang * A new thin film buckling system was developed from photosensitive SU-8 thin films doped with UV light absorbing dye, resulting in a depth-wise gradient modulus after photocrosslinking. When the film was swollen by an organic solvent, a wide range of wrinkling patterns was obtained, including lamella, peanut and semi-hexagonal patterns. Both the morphology and wavelength were found to be dependent on the original film thickness. By leveraging the thermoplasticity of SU-8, we imprinted one- dimensional (1-D) patterns on the dyed SU-8 with variable pitch and height from 1 mm to 20 mm and 15 nm to 2 mm, respectively. We then swelled the patterned films and investigated the interactions between the intrinsic buckling waves (both size and morphology) and the pre-patterns. As the pre- pattern pitch decreased, the swollen film in the patterned region evolved from isotropic wrinkles to out- of-phase, anisotropic waves, which further became in-phase when the pre-pattern pitch was smaller than the intrinsic wrinkle wavelength. For the latter, the aligned wrinkle morphology varied dramatically when the pre-pattern height decreased: from perpendicular to the pre-pattern wavevector to dual orientation with one set of wrinkles remained perpendicularly ordered and the other set of local buckling patterns aligned in parallel to the pre-pattern, and finally back to isotropic ones. Since the pre- patterns of different size and shape could be readily prepared, the combination of physical confinement together with controlled swelling in a graded thin film offers a new approach to access a wide range of controllable hierarchical patterns. Introduction Wrinkling in thin lms, including bi-layers 1–14 and layers with crosslinking gradient, 15–17 is of interest for a wide variety of applications, including organic light-emitting diodes, 5 exible electronics, 18,19 microlens arrays, 9,20 adhesives 21,22 and thin lm metrology. 23 Depending on the type and conditions of the stress induced, pattern morphology and dimensions can be ne- tuned with wavelength spanning from 0.1 to 100 mm when the stress exceeds the critical buckling stress. Mechanical 6,7,24 and osmotic stresses 8–12,21,25,26 are commonly utilized to generate wrinkles. Typically, random wrinkle morphology is formed on at 1,2,5,14,15,17,24 and curved lms 27,28 under an isotropic stress (e.g. thermal annealing and solvent swelling/annealing). For most applications, however, ordered structures 9,18,19,22 are desired. It has been demonstrated that one-dimension (1-D) ripples and two-dimension (2-D) zigzag wrinkles can be created by controlling the level and direction of applied mechanical strain. 6,29–32 In the case of heating/cooling or solvent swelling/ annealing of a patterned lm, 2–4,10,11,26,33–35 the growing wrinkles can be aligned with the pattern edges due to the stress relaxa- tion perpendicular to the propagating swelling wave front. 13 This alignment persists to a certain distance before the wrinkles become isotropic again. This persistence length is typically much larger than the intrinsic wavelength, which is determined by the stress state and the respective Young's modulus of the lm and substrate. 7,9,12,26 It has been suggested that when conning the wrinkle with a template that is comparable to or smaller than the persistent length, morphological alignment can be induced, where the wrinkle morphology depends on both the boundary geometry and the stress state. 2,3,9,26 So far, experimentally there have been only a few scattered results from pre-patterns with feature size smaller than the wrinkle wave- length or amplitude. 2,32,36,37 For example, Okayasu et al. 2 pre- sented preliminary results on small pattern effects on thermally induced wrinkles but the mechanism was not well-understood and the aligned wrinkles did not show long-range in-phase order. Ohzono et al. 38,39 and Lee et al. 32 have also studied wrinkle formation on small subsumed patterns. However, for both of their systems, the wrinkles were mechanically generated whose Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, PA 19104, USA. E-mail: shuyang@seas.upenn.edu; Fax: +1-215-573-2128; Tel: +1-215-898-9645 † Electronic supplementary information (ESI) available: The relative transmittance of 3.8 mm thick dyed SU-8; the swelling behavior of a free-standing, dyed SU-8 lm; early stage of buckling of the pre-patterned ( p ¼ 20 mm, h ¼ 800 nm), dyed SU-8; schematic of the shiing of the neutral plane in the presence of pre-patterns. See DOI: 10.1039/c3sm51881g Cite this: Soft Matter, 2013, 9, 11007 Received 10th July 2013 Accepted 19th September 2013 DOI: 10.1039/c3sm51881g www.rsc.org/softmatter This journal is ª The Royal Society of Chemistry 2013 Soft Matter , 2013, 9, 11007–11013 | 11007 Soft Matter PAPER