Abstract
Wrinkles, as a form of mechanical instability, can be employed in many different areas, including self-cleaning coating, flexible electronics, smart adhesion, channel for microfluidic devices, and mechanical property measurements. After two decades of research, the wavelength and amplitude of wrinkles can be well controlled by proper design of physical properties of thin films and substrates. However, it is still very difficult to change their orientations and complexity in a single piece of thin film, which is critically important for many practical applications. In previous studies, the orientation of all the wrinkles are either along the same direction or can only be controlled to some extent by the patterning of relief structures which rely on the complicated lithography techniques and cannot be adjusted for a given substrate. Here, we employed the shape memory polymer (SMP) as a smart substrate, combined with local/selective heating to control the boundary conditions for the strain field, and realized the gradual change in the orientation of wrinkles in a single piece of thin film. The wrinkles with a gradual change in orientations exhibited angle-dependent colors. Furthermore, by changing the sequence of thin film coating and partial triggering of an SMP, complex surface features with a sharp interface can be obtained. Finite element simulation investigations uncover the basic principles and requirements that need to be satisfied, as well as the limitations of our method, to generate wrinkles with controlled orientations in a single piece of thin film.
Highlights
Wrinkles are ubiquitous in nature as a form of mechanical instability, occurring in vast different systems ranging from animal skin to lava flow.1,2 They were previously treated as a nuisance to be avoided, but in recent years, wrinkles have been taken advantage as exquisite patterns for advanced applications.3–5 For example, wrinkles can be employed to measure the properties of a thin film, enhance light extraction, separate small particles, endow brittle thin films with stretchability, and act as the channels for the solution flow in microfluidic devices, just to name a few.6–9From the perspective of mechanics, wrinkling is essentially a type of structure instability, and wrinkles can be formed in a piece of thin film coated on prestretched rubber or a shape memory polymer (SMP) substrate
We further show that different sequences of SMP recovery and thin film deposition can even result in complex surface features with a sharp boundary
The principal stresses (PSs) in both x and y directions are well aligned at all locations
Summary
Wrinkles are ubiquitous in nature as a form of mechanical instability, occurring in vast different systems ranging from animal skin to lava flow. They were previously treated as a nuisance to be avoided, but in recent years, wrinkles have been taken advantage as exquisite patterns for advanced applications. For example, wrinkles can be employed to measure the properties of a thin film, enhance light extraction, separate small particles, endow brittle thin films with stretchability, and act as the channels for the solution flow in microfluidic devices, just to name a few.. Wrinkles are ubiquitous in nature as a form of mechanical instability, occurring in vast different systems ranging from animal skin to lava flow.. Wrinkles are ubiquitous in nature as a form of mechanical instability, occurring in vast different systems ranging from animal skin to lava flow.1,2 They were previously treated as a nuisance to be avoided, but in recent years, wrinkles have been taken advantage as exquisite patterns for advanced applications.. Developing a new strategy, by which the wrinkles can be patterned at any location as desired and their orientation can be controlled or changed at will in a single piece of thin film, is very important for practical applications, such as structure colors, microfluidic devices, and templates for nanowire growth. We focus on the patterning and orientation control of the wrinkles by heating an SMP substrate locally [Fig. 1(b)]. Basic principles and requirements that need to be satisfied, as well as the limitations of our method, to generate wrinkles with controlled orientations in a single piece of thin film are investigated by a finite element method (FEM)
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