Wageningen University researchers: Eric van Bruggen, Erik van der Linden and Mehdi Habibi from the Laboratory of Physics and Physical Chemistry of Foods recently have shown how ductility and friction can be used to tailor the relaxation dynamics of crumpled metamaterials. Using a simple experimental setup and a theoretical framework the researchers found surprising results, such as the fact that by increasing the adhesion (/friction) between the crumpled layers the relaxation time decreases. Their research work is currently published in the research journal, Soft Matter. According to Dr. Habibi: “origami structures have recently attracted much interest for designing metamaterials with desired mechanical properties. However, all ordered structures are constrained by geometric relations. These ordered geometries can be easily violated in real life by defects which strongly affecting their mechanical functionality. On the other hand, crumpling is a more robust method for making 3D structures, and because of random geometric configurations of crumpled structures, they are inherently insensitive to defects and noise. Then the main question would be how to manipulate the mechanical properties of the random crumpled structures for designing mechanical metamaterial. Our research reveals that by tuning the friction and adhesion of the crumpled layers we will be able to tailor the mechanical response of the material.”
Studying crumpled materials have attracted a lot of attention in recent years. This can be attributed to their excellent mechanical energy absorption and high mechanical strength that make them suitable for numerous application such as shock absorbers. Interestingly, recent studies have shown slow mechanical relaxation and memory of crumpled materials which up till now have negatively affected their applications. As such, physical understanding of crumpled materials properties is highly desirable and especially discovering the mechanisms behind the slow stress relaxation.
In brief, the research team initiated their research work by preparing different crumpled sheets of different materials and with different ductility properties and friction coefficients. Secondly, a double logarithmic model was developed to analyse stress relaxation in crumpled structures. In addition, the aging behavior of the various materials investigated, was analyzed and compared to each other using a two-step compression protocol, while taking into consideration their relaxation dynamics. This was done particularly for elastomeric, polymeric and metal sheets.
Based on this research now it is possible to tune the material relaxation dynamics by varying ductility and adhesive properties of the material. The authors observed that factors affecting the relaxation dynamics included: the properties of the materials (ductility and elastoplasticity), adhesion and friction coefficient. All the materials exhibited non-monotonic aging, thus confirming that the behaviour of crumpled systems is dependent on the compaction protocol. In addition, the height of the aging peak, linearly depended on the rising time with respect to the slope.
In summary, the Wageningen University scientists managed to look carefully at the factors influencing the structural relaxation and mechanical memory of crumpled materials. The main factors were identified as ductility, friction, and adhesion. To actualize their study, they compared the stress relaxation curves for the materials tested. Interestingly, the obtained data was in line with the popularly used exponential and stretched models. Altogether, their study is important because it provides vital information that will advance the optimization of crumpled materials to obtain the desired properties for various applications.
van Bruggen, E., van der Linden, E., & Habibi, M. (2019). Tailoring relaxation dynamics and mechanical memory of crumpled materials by friction and ductility. Soft Matter, 15(7), 1633-1639.Go To Soft Matter