<p>Wrinkling phenomenon in which a flat surface deforms into periodic wavy patterns is frequently observed in nature such as brain cortex, flower petal, and human skin. Such a mechanical instability has also been applied in artificial systems through strain engineering to improve interfacial properties such as wettability and optic reflectivity. Although extensive studies have been performed to identify the mechanics of wrinkles, a reliable strategy to dynamically control the wrinkle’s topology for tunable adhesion through electric signals remains a challenge. In this work, we designed a bilayer, smart wrinkled surface in which one thin layer of smart composite is attached to a stretched Polydimethylsiloxane (PDMS) substrate. The smart composite consists of three components including a PDMS matrix, Field’s metal particles and nickel-coated carbon fibers and is synthesized through mixing and heat pressing. Owing to the liquid-solid phase transition of the Field’s metal particles at relatively low melting point (65 C), the Young’s modulus of the smart composite decreases significantly with Joule heating generated by the electric current. By exploiting the stiffness change of such a smart composite, we can vary the wrinkle’s topology by applying electric current to the top film of the bilayer structure, since the wrinkle’s wavelength and amplitude depends on the modulus ratio between the top film and the substrate as predicted by the classical theory. In experiments, we fabricate such a PDMS-composite bilayer structure and integrate it with electric wires so that we can activate the sample with electric currents. White light interferometry is used to measure the wrinkle’s topology with and without electric current, and we observe a decrease of the wrinkle’s amplitude when the electric signal is applied. The adhesion property of the wrinkle surface is measured by the pull-off force of the sample against a flat PDMS block using an Instron machine, and a systematic variation of the pull-off force is observed when the bilayer structure is activated. Cyclic tests are also performed, and no degradation of the device’s performance is observed after dozens of cycles of the electric signals. In addition to the experiments, we also performed finite element (FE) simulations on the bilayer structure based on the “model change” command in commercial software ABAQUS, and good agreement is obtained between the numerical and experimental results regarding the wrinkle’s topology. Our results build a new prototype of smart wrinkled surface for tunable adhesion through electric signals, which can have potential applications in robotic systems for various functionalities such as gripping and locomotion. &nbsp;</p>

Smart Wrinkled Surfaces for Tunable Adhesion Enabled by Composites With Tunable Stiffness

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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