Reversible wrinkling composites
Taking inspiration from examples in nature, researchers at the Massachusetts Institute of Technology (MIT) have identified the mechanics involved in wrinkling in response to certain stimuli, aiding the future development of ‘smart’ composites with wrinkled microstructures. It is hoped that the understanding gained about the natural mechanisms involved in internal wrinkling will help scientists create responsive materials with potential applications in chemical sensing, medical diagnostics and optical and acoustic wave control.
The researchers focussed their efforts on what they deemed an ‘ideal’ structure – a series of interfacial layers within soft-layered composites. This is similar to the configuration seen in arteries and cell walls, and allows for reversible wrinkling. In essence, this structure consists of layers of a soft, rubbery material separated by thin layers of a relatively stiff material. When compressed, the stiff, thin layers may buckle or wrinkle, depending upon the force applied. Removing the force causes the rubbery layers to spring back into shape, straightening out the thinner layers.
Taking into account the stiffness of the layers, the geometry and the load applied, the researchers managed to develop a model that predicts the wavelength and amplitude that wrinkles will take once a critical load is reached. The group’s model is detailed in the journal Advanced Engineering Materials.
‘Based on the materials and geometry of the composite structure, the equations we’ve developed predict whether the interfacial layers will wrinkle or not, and the wrinkling pattern that might be achieved,’ said Narges Kaynia, a graduate student in mechanical engineering at MIT. ‘It’s very fundamental knowledge that has a lot of applications.’
Kaynia collaborated with Mary Boyce, the Ford Professor of Mechanical Engineering and head of MIT’s Department of Mechanical Engineering, and Yaning Li, a former MIT postdoc who is now an assistant professor of mechanical engineering at the University of New Hampshire.
The model was used to correctly predict the wrinkling and instability pattern of composite materials that were made using multi-material, three-dimensional printing. According to Li and Kaynia, the model may be used to help scientists engineer biomimetic materials with interfacial layers that reversibly wrinkle on demand. These might wrinkle in response to mechanical, chemical or optical stimuli.
‘The [wrinkling] of materials can generate new functionalities that have never been achieved before,’ said Yonggang Huang, a professor of civil and mechanical engineering at Northwestern University. ‘The authors have studied this important phenomenon [and] this is the first demonstration of reversible wrinkling of interfaces in multilayered materials.’
This research was supported by the MIT Center for Materials Science and Engineering through a grant from the National Science Foundation.
Photograph by Narges Kaynia and Yaning Li, courtesy of MIT.