Haipeng Ji

Tuning and Erasing Surface Wrinkles by Reversible Visible-Light-Induced Photoisomerization.

Periodic wrinkling across different scales has received considerable attention because it not only represents structure failure but also finds wide applications. How to prevent wrinkling or create desired wrinkling patterns is non-trivial because the dynamic evolution of wrinkles is a highly nonlinear problem. Herein, we report a simple yet powerful method to dynamically tune and/or erase wrinkling patterns with visible light. The light-induced photoisomerization of azobenzene units in azopolymer films leads to stress release and consequently to the erasure of the wrinkles. The wrinkles in unexposed regions are also affected and oriented perpendicular to the exposed boundary during the stress reorganization. Theoretical models were developed to understand the dynamics of the reversible photoisomerization-induced wrinkle evolution. This method can be applied for designing functional materials/devices, for example, for the reversible optical writing/erasure of information as demonstrated here.

Bioinspired Fabrication of Free-Standing Conducting Films with Hierarchical Surface Wrinkling Patterns

Mechanical instability has been shown to play an important role in the formation of wrinkle structures in biofilms, which not only can adopt instability modes as templates to regulate their 3D architectures but also can tune internal stresses to achieve stable patterns. Inspired by nature, we report a mechanical-chemical coupling method to fabricate free-standing conducting films with instability-driven hierarchical micro/nanostructured patterns. When polypyrrole (PPy) film is grown on an elastic substrate via chemical oxidation polymerization, differential growth along with in situ self-reinforcing effect induces stable wrinkle patterns with different scales of wavelengths. The self-reinforcing effect modifies the internal stresses, hence PPy films with intact wrinkles can be removed from substrates and further transferred onto target substrates for functional device fabrication. To understand the buckling mechanics, we construct a model which reveals the formation of hierarchical wrinkle patterns.

Redox-Switchable Surface Wrinkling on Polyaniline Film.

Here the redox-driven switch between the wrinkled and dewrinkled states on poly-aniline (PANI) film is reported. This switch is derived from the reversible transition in different intrinsic redox states of polyaniline (e.g., between emeraldine salt (ES) and leucoemeraldine base (LEB) or between ES and pernigraniline base (PB)) that are involved in the redox reaction, coupled with the corresponding volume expansion/shrinkage. Interestingly, the as-wrinkled ES film becomes deswollen and dewrinkled when reduced to the LEB state or oxidized to the PB state. Conversely, oxidation of the LEB film or reduction of the PB film into the swollen ES film leads to the reoccurrence of surface wrinkling. Furthermore, the reducibility of the dewrinkled LEB film and the oxidizability of the dewrinkled PB film are well utilized respectively to yield various wrinkled PANI-based composite films.

Patterning Surfaces on Azo-Based Multilayer Films via Surface Wrinkling Combined with Visible Light Irradiation

Here, a simple combined strategy of surface wrinkling with visible light irradiation to fabricate well tunable hierarchical surface patterns on azo-containing multilayer films is reported. The key to tailor surface patterns is to introduce a photosensitive poly(disperse orange 3) intermediate layer into the film/substrate wrinkling system, in which the modulus decrease is induced by the reversible photoisomerization. The existence of a photoinert top layer prevents the photoisomerization-induced stress release in the intermediate layer to some extent. Consequently, the as-formed wrinkling patterns can be modulated over a large area by light irradiation. Interestingly, in the case of selective exposure, the wrinkle wavelength in the exposed region decreases, while the wrinkles in the unexposed region are evolved into highly oriented wrinkles with the orientation perpendicular to the exposed/unexposed boundary. Compared with traditional single layer-based film/substrate systems, the multilayer system consisting of the photosensitive intermediate layer offers unprecedented advantages in the patterning controllability/universality. As demonstrated here, this simple and versatile strategy can be conveniently extended to functional multilayer systems for the creation of prescribed hierarchical surface patterns with optically tailored microstructures.

Surface treatment-assisted switchable transfer printing on polydimethylsiloxane films

Here, we report simple novel surface treatment-assisted switchable transfer printing (sTP) to pattern polydimethylsiloxane (PDMS) films of diverse thicknesses with prescribed microstructures. In the sTP, the PDMS decal inks are transferred from the “soft” side to the “hard” side, irrespective of the “soft” side coming from the PDMS stamp or the target PDMS substrate. Their “soft/hard” contrast with the corresponding cohesion difference can be finely induced and switched by a simple surface oxidation treatment, yielding the customized transfer patterns. In particular, multiple sTP with smart combinations of the additive/subtractive transfer mode can be easily realized for the controlled fabrication of unprecedented hierarchical patterns. We demonstrate the application of this methodology for forming microfluidic systems and as masks for surface patterning, and it is a powerful candidate for photolithography. Systematic studies further provide insights into the involved cohesion–fracture mechanism, which is highly expected to apply to other functional systems for advanced micro–nanofabrication.

Self-Supported Crack-Free Conducting Polymer Films with Stabilized Wrinkling Patterns and Their Applications

Self-supported conducting polymer films with controlled microarchitectures are highly attractive from fundamental and applied points of view. Here a versatile strategy is demonstrated to fabricate thin free-standing crack-free polyaniline (PANI)-based films with stable wrinkling patterns. It is based on oxidization polymerization of pyrrole inside a pre-wrinkled PANI film, in which the wrinkled PANI film is used both as a template and oxidizing agent for the first time. The subsequently grown polypyrrole (PPy) and the formation of interpenetrated PANI/PPy networks play a decisive role in enhancing the film integrity and the stability of wrinkles. This enhancing effect is attributed to the modification of internal stresses by the interpenetrated PANI/PPy microstructures. Consequently, a crack-free film with stable controlled wrinkles such as the wavelength, orientation and spatial location has been achieved. Moreover, the wrinkling PANI/PPy film can be removed from the initially deposited substrate to become free-standing. It can be further transferred onto target substrates to fabricate hierarchical patterns and functional devices such as flexible electrodes, gas sensors, and surface-enhanced Raman scattering substrates. This simple universal enhancing strategy has been extended to fabrication of other PANI-based composite systems with crack-free film integrity and stabilized surface patterns, irrespective of pattern types and film geometries.

Simple and Versatile Strategy to Prevent Surface Wrinkling by Visible Light Irradiation.

A stiff film bonded to a compliant substrate is susceptible to surface wrinkling when it is subjected to in-plane compression. Prevention of surface wrinkling is essential in many cases to maintain the integrity and functionality of this kind of system. Here we report a simple versatile technique to restrain surface wrinkling of an amorphous poly(p-aminoazobenzene) (PAAB) film by visible light irradiation. The key idea is to use the combined effects of photosoftening of the PAAB film and the stress release induced by the reversible photoisomerization. The main finding given by experiments and dimensional analysis is that the elastic modulus Ef of the film is well modulated by the ratio of light intensity and the release rate, i.e., I/V. Furthermore, the explicit solution describing the correlation of I/V with Ef is derived for the first time. The difference between the calculated critical wrinkling strain εc,t based on Ef and the experimentally measured value εc enables us to quantitatively evaluate the release amount of the compressive stress in the film. These key solutions provide a simple strategy to prevent the undesired surface wrinkling. Additionally, they allow us to propose a wrinkling-based technique to investigate photoinduced changes in the mechanical properties of azo-containing materials.

Ordered arrays of pumpkin-shaped Janus particles with tailored surface morphologies via microcontact hot embossing

In this paper, we report the controlled fabrication of ordered arrays of pumpkin-shaped (i.e., shapes having the equatorial diameter greater than the polar diameter and being flattened at the poles) Janus polystyrene (PSt) particles with tailored morphologies. It is based on simple microcontact hot embossing of monodispersed PSt microsphere monolayers using different stamps such as glass slides, colloidal sphere monolayers, and wrinkled sheets. The resultant pumpkin-shaped particles have the stamp-defined surface topographies, with one side flat and the other flat or patterned with small beads/holes and striped/dotted bulges. We systematically investigate the effect of the embossing conditions on the surface morphologies, which include the annealing temperature/time, the external load, and the structured stamps. Benefiting from the hexagonal close packing of original symmetric microspheres, well-ordered Janus particle arrays are obtained using the combined bottom-up/top-down strategy. Compared with the traditional two-stage process of fabrication and self-assembly, this method is much simpler and more straightforward, which is of significant importance for the related applications.