Elasticity of Heterogeneous Gels

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Gels are three-dimensional polymer networks capable of absorbing a large amount of solvent molecules subject to various external stimuli (pH, temperature, light, etc.). They exhibit a rich mechanical behavior and prominent nonlinearity owing to their high flexibility, stimuli-responsiveness and superabsorbency. More compelling are the intriguing morphologies and novel functionalities achieved by introducing mechanical heterogeneities to an otherwise homogeneous gel. The misfit between heterogeneous components can cause mechanical instabilities that generate complex shapes such as creases, wrinkles, folds and helices. These buckling structures have broad engineering applications, and are also important model systems to understand the shape generation in biological bodies. Additionally, microstructural heterogeneities incorporated into the original gel networks can endow the gel with strong mechanical anisotropy, high toughness and high modulus. Such composite gels are particularly attractive as novel biomaterials due to their structural similarities to many biological systems in nature. To understand the large deformation behavior and mechanical instabilities of gels with heterogeneities, we employ a finite element approach to investigate three systems with different architectures. The first system regards to a complex contact deformation of an elastomeric pyramid array which is widely adopted in advanced nanopatterning techniques. Simple scaling laws of the deformation are established and compared with existing experiments. We further show that the distinct deformed shape of the pyramid plays a decisive role in producing the previously unexplained photoresist patterns. In the second system, the mechanical instability in a simple heterogeneous structure is considered. In a bistrip gel with different prestrains in each strip, perversions and helices can emerge when the ends of the bistrip approach with each other. Perversions serve as as generic domain walls that connect states of opposite chirality. Here we focus on numerical analysis of the intrinsic properties of perversions, including the strain energy condensation over perversions, the repulsive nature of the perversion-perversion interaction and the coalescence of perversions. These findings have implications to the understanding of relevant biological motifs. Finally, we explore the anisotropic contraction of hydrogels reinforced by aligned fibrous heterogeneities, inspired by the recent experimental work of Chin et. al. Several strategies are proposed to improve the contraction anisotropy based on Flory-Rehner theory and finite element simulations. The numerical analysis indicates an increasing of contraction anisotropy when the hydrogel is prestretched along the fiber direction. Simulations further show that the contraction anisotropy can be maximized by tuning the structure parameters of the embedded fibers.

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  • 02/08/2019
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