On the Formulation and the Micromechanical Origin of Non-Classical Models of Diffusion

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Diffusion processes are of utmost importance for a variety of applications in engineering and natural sciences. Drug transport in biological tissue, charge and discharge cycles in batteries, or the formation of microscructures in alloys are just a few examples. There, however, the classical diffusion model of Fickian-type often fails in explaining the complex nature of these phenomena and thus, more sophisticated non-classical diffusion models are employed. In view of the diversity of these models, the overarching goal of the project is thereby the clarification of the micromechanical origin of non-classical diffusion models. Therefore, a generic class of first and second gradient-type rigid diffusors and micromorphic-type diffusors has been formulated and implemented in the first project phase using different computational methods, e.g. finite elements, natural elements, and isogeometric analysis. Based on that, computational homogenization is and will further be carried out to determine the unknown macroscale constitutive response from the constitutive response at the microscale. Dictated by the characteristic length of the underlying microstructure, we consider the microscale problem to be either stationary or instationary, which induces size effects to the macroscale solution. To better account for small length scales at the micro-level, we propose for the second project phase the introduction and formulation of energetic interfaces at the microscale to describe phenomena like anomalous diffusion along or across an interface or surface tension when coupled to deformation. In numerous engineering applications, diffusion accompanies deformation, for which reason their coupling is particularly important and will be intensively studied within the framework of computational homogenization. Moreover, to further elucidate the micromechanics of diffusion processes we plan to employ a discrete model and identify its impact on the macroscale solution. We believe that the outcome of the of this project is of particular importance in engineering and material sciences, e.g. for the development of new materials or joining processes.