Extremely Soft: Design with Rheologically Complex Fluids

Soft is a relative term. Its scope includes the extreme of viscoelastic materials that exhibit both liquid-like and solid-like properties depending on loading conditions. Such rheologically complex materials are abundant in biological organisms, including mucin gels, blood, cellular cytoplasm, and molecules of protein or DNA. However, comparably soft, rheologically complex materials are not yet in the standard toolbox of the engineer. Engineering design generally, and robotic design specifically, stand to benefit greatly from soft and rheologically complex materials that could offer novel performance based on nonlinear material properties. In order to achieve this, several technical challenges must be overcome, including better conceptual modeling, more predictive mathematical modeling, and an overall design framework that manages the high dimensionality of rheological material properties. This article outlines the challenges, and as a first step toward overcoming them, describes a design process that manages rheological complexity in the relationship between performance, function-valued properties, and material formulation.