Tailoring Ordered Wrinkle Arrays for Tunable Surface Performances by Template-Modulated Gradient Films

Smart Wrinkled Interfaces: Patterning, Morphing, and Coding of Polymer Surfaces by Dynamic Anisotropic Wrinkling

In contrast to traditional static surfaces, smart patterned surfaces with periodical and reversible morphologies offer limitless opportunities for encoding surface functions and properties on demand, facilitating their widespread application as functional building blocks in various devices. Advances in intelligently controlling the macroscopic properties of these smart surfaces have been accomplished through various techniques (such as three-dimensional printing, imprint lithography and femtosecond laser) and responsive materials. In contrast to the sophisticated techniques above, dynamic anisotropic wrinkling, taking advantage of dynamic programmable manipulation of surface wrinkling and its orientation, offers a powerful alternative for fabricating dynamic periodical patterns due to its spontaneous formation, versatility, convenient scale-up fabrication, and sensitivity to various stimuli. This review comprehensively summarizes recent advances in smart patterned surfaces with dynamic oriented wrinkles, covering design principles, fabrication techniques, representative types of physical and chemical stimuli, as well as fine-tuning of wrinkle dimensions and orientation. Finally, advanced applications of these smart patterned surfaces are presented, along with a discussion of current challenges and future prospects in this rapidly evolving field. This review would offer some insights and guidelines for designing and engineering novel stimuli-responsive smart wrinkled surfaces, thereby facilitating their sustainable development and progressing toward commercialization.

Microelectronic Morphogenesis: Smart Materials with Electronics Assembling into Artificial Organisms

Microelectronic morphogenesis is the creation and maintenance of complex functional structures by microelectronic information within shape-changing materials. Only recently has in-built information technology begun to be used to reshape materials and their functions in three dimensions to form smart microdevices and microrobots. Electronic information that controls morphology is inheritable like its biological counterpart, genetic information, and is set to open new vistas of technology leading to artificial organisms when coupled with modular design and self-assembly that can make reversible microscopic electrical connections. Three core capabilities of cells in organisms, self-maintenance (homeostatic metabolism utilizing free energy), self-containment (distinguishing self from nonself), and self-reproduction (cell division with inherited properties), once well out of reach for technology, are now within the grasp of information-directed materials. Construction-aware electronics can be used to proof-read and initiate game-changing error correction in microelectronic self-assembly. Furthermore, noncontact communication and electronically supported learning enable one to implement guided self-assembly and enhance functionality. Here, the fundamental breakthroughs that have opened the pathway to this prospective path are reviewed, the extent and way in which the core properties of life can be addressed are analyzed, and the potential and indeed necessity of such technology for sustainable high technology in society is discussed.

Tunable Magnetic Domain Patterns in Thickness-Gradient Nickel Films on Flexible PDMS Substrates

Flexible magnetoelectronic devices (based on magnetic films) have great application prospects in the fields of information storages, bionic robotics, and electronic skins. The intrinsic stress and external loading are very important to modulate the structures and properties of flexible magnetic films due to the magnetoelastic coupling effect. Here, we report on tunable magnetic domain patterns in thickness-gradient nickel (Ni) films deposited on flexible polydimethylsiloxane substrates. It is found that stripe magnetic domains spontaneously form in the Ni films and their sizes increase with the film thickness. The internal stress evolves from tensile to compressive states with decreasing film thickness, leading to the formation of cracks in thicker regions and wrinkles in thinner regions. Meanwhile, the orientations of stripe magnetic domains change from the gradient direction to the orthogonal direction. The structural features, evolution behaviors, and physical mechanisms of the cracks, wrinkles, and magnetic domains are analyzed based on the stress theory and magnetoelastic coupling. Periodic gradient Ni films with large-scale regulations of stripe magnetic domains are also realized by masking of copper grids. This study helps to better understand the magnetoelastic coupling effect in gradient flexible magnetic films and provides a technique to modulate anisotropic magnetic properties by designing specific film systems.

Wrinkled Thermo-Electric Meander-Shaped Element on a Thin Freestanding PDMS Membrane

Natural wrinkling of metal films on silicone substrates can appear by means of the metal sputtering process and can be described by the continuous elastic theory and non-linear wrinkling model. Here, we report the fabrication technology and behavior of thin freestanding Polydimethylsiloxane (PDMS) membranes equipped with thermo-electric meander-shaped elements. The Cr/Au wires were obtained on the silicone substrate by magnetron sputtering. We observe wrinkle formation and suppose furrows appear once PDMS returns to its initial state after the thermo-mechanical expansion during sputtering. Although the substrate thickness is usually a negligible parameter in the theory of wrinkle formation, we found that the self-assembled wrinkling architecture of the PDMS/Cr/Au varies due to the membrane thickness of 20 µm and 40 µm PDMS. We also demonstrate that the wrinkling of the meander wire affects its length, and it causes a 2.7 times higher resistance compared to a calculated value. Therefore, we investigate the influence of the PDMS mixing ratio on the thermo-electric meander-shaped elements. For the stiffer PDMS with a mixing ratio of 10:4, the resistance due to wrinkle amplitude alterations is 25% higher compared to the PDMS of ratio 10:1. Additionally, we observe and describe a thermo-mechanically induced motion behavior of the meander wires on completely freestanding PDMS membrane under applied current. These results can improve the understanding of wrinkle formation, which influences thermo-electric characteristics and may promote the integration of this technology in applications.

Superrepellent Porous Polymer Surfaces by Replication from Wrinkled Polydimethylsiloxane/Parylene F

Superrepellent surfaces, such as micro/nanostructured surfaces, are of key importance in both academia and industry for emerging applications in areas such as self-cleaning, drag reduction, and oil repellence. Engineering these surfaces is achieved through the combination of the required surface topography, such as porosity, with low-surface-energy materials. The surface topography is crucial for achieving high liquid repellence and low roll-off angles. In general, the combination of micro- and nanostructures is most promising in achieving high repellence. In this work, we report the enhancement of wetting properties of porous polymers by replication from wrinkled Parylene F (PF)-coated polydimethylsiloxane (PDMS). Fluorinated polymer foam “Fluoropor” serves as the low-surface-energy polymer. The wrinkled molds are achieved via the deposition of a thin PF layer onto the soft PDMS substrates. Through consecutive supercritical drying, superrepellent surfaces with a high surface porosity and a high water contact angle (CA) of >165° are achieved. The replicated surfaces show low roll-off angles (ROA) <10° for water and <21° for ethylene glycol. Moreover, the introduction of the micro-wrinkles to Fluoropor not only enhances its liquid repellence for water and ethylene glycol but also for liquids with low surface tension, such as n-hexadecane.

Controllable Buckle Delaminations in Polymer-Supported Periodic Gradient Films by Mechanical Compression

Surface instabilities including wrinkles and buckle-delaminations are widespread in nature and can be found in a wide range of practical applications. Compared with the homogeneous wrinkle mode, the buckle-delaminations are spontaneously stress-localized, and their initiation positions and geometrical parameters are hardly precisely controlled by a simple method. Here, we report on the controllable buckle-delaminations in periodic thickness-gradient metal films on polydimethylsiloxane (PDMS) substrates by uniaxial mechanical compression. It is found that a periodic thickness-gradient film is spontaneously formed by masking a copper grid during deposition. The released mechanical strain tends to concentrate in thinner film regions, resulting in the restricted growth of buckle-delaminations. The geometrical features, evolutional behaviors, and underlying physical mechanisms of such buckle-delaminations are analyzed and discussed in detail based on the buckling model and finite element simulations. This work would provide a better understanding of the restricted buckle-delaminations in heterogeneous film-substrate systems and controllable fabrication of ordered structural arrays by copper grid masking and mechanical loading.

Reversible Wrinkling Surfaces for Enhanced Grip on Wet/Dry Conditions

Friction is important in material design for robotic systems that need to perform tasks regardless of environmental changes. Generally, robotic systems lose their friction in wet environments and fail to accomplish their tasks. Despite the significance of maintaining friction in dry and wet environments, it is still challenging. Here, we report a smart switching surface, which helps to complete missions in both wet and dry environments. Inspired by the reversible wrinkling mechanism of a human finger, the surface reversibly generates and removes wrinkles to adapt to both environments using volume-changing characteristics of the Nafion film. The switchable surfaces with manipulated wrinkle morphologies via patterns of diverse densities, sizes, and shapes induce a relationship between the wrinkle morphologies and friction: wrinkles on denser and smaller hexagonal patterns generate six times more friction than non-switching flat surfaces in wet environments and a similar amount of friction to the flat surfaces in dry environments. In addition, the wrinkle morphologies according to the patterns are predicted through numerical simulation, which is in good agreement with experimental results. This work presents potential applications in robotic systems that are required to perform in and out of water and paves the way for further understanding of wrinkling dynamics, manipulation, and evolutionary function in skin.

Dynamic Surface Wrinkles for In Situ Light-Driven Dynamic Gratings

Dynamic diffraction gratings (DDGs) are considered as one of the most promising technologies for application in smart optical devices because of their in situ dynamic regulation of light propagation on demand; however, it is still a challenge to fabricate dynamic periodic micro/nanostructures due to limited materials and processes. Here, a facile and feasible strategy to construct a near-infrared (NIR) radiation-driven DDG is developed based on a double-sided surface pattern, which is fabricated by dynamic wrinkles and/or soft-imprinted static wrinkles. Poly(dimethylsiloxane) (PDMS) containing carbon nanotubes (CNTs) serves as the substrate, and wrinkles are formed on both sides. The resulting double-sided wrinkle pattern can be used as a DDG to generate various adjustable two-dimensional (2D) diffraction patterns driven by NIR light. Furthermore, with various combinations of wrinkles, we demonstrated a single-sided responsive DDG and a double-sided responsive DDG to realize the evolution of diffraction patterns from 2D to one-dimensional (1D) and 2D to zero-dimensional (0D), respectively. The results provide an alternative for DDGs that will have wide applications in smart display, sensing, and imaging systems.

Surface Wrinkling on Polymer Films

A series of gold precursor solutions are prepared by dissolving HAuCl₄ and its mixtures with K₂CO₃ of different contents in deionized (DI) water. Neat HAuCl₄ predominately forms AuCl₄^- ions in an aqueous solution. In the presence of K₂CO₃, AuCl₄^- ions hydrolyze to form [AuCl_4-x(OH)_x]^- complex ions. Increasing the content of K₂CO₃ in a gold precursor solution increases the content of [AuCl_4-x(OH)_x]^- complex ions and decreases the content of AuCl₄^- ions. Poly(4-vinyl pyridine) (P4VP) films of two different molecular weights are deposited on SiO_x/Si by spin coating, by which the thicknesses are controlled by polymer weight fractions in butanol. Those P4VP films form periodic wrinkles when immersed in aqueous solutions, followed by drying. The surface wrinkling is induced by swelling pressure that overwhelms the mechanical property of the P4VP film. The periodicity and amplitude of wrinkles grown on the P4VP films strongly correlate with initial thickness, AuCl₄^- ion content, and residual stress.