Multiphase Soft Materials: Local Structuring Sets Global Mechanical Behavior

Abstract

Multiphase materials are composed of two or more constituents of different phases whose characteristics can vary widely, leading to a multi-component system with properties that are greater than the sum of its parts. The mechanical behavior of multiphase materials depends on the mesoscale structure of the material, which itself can be affected by the microscopic interactions between the constituents. Multiphase materials are ubiquitous; examples include toothpaste, sandcastles, or photographic film. We here study the macroscopic behavior of multiphase soft materials, focusing on two systems: nanofiller-reinforced composite hydrogels and colloidal gels. Hydrogels and colloidal gels are promising functionalized materials in the biomedical field due to their high water content and biocompatibility. Predicting the behavior of these soft materials under deformation or load is key to fine-tuning their use to specific applications, and relies on elucidating the link between microscopic inter- and intra-phase interactions and macroscopic rheological properties. We present a comprehensive characterization of the mechanical properties of multiphase soft materials, from their linear elastic response to their yielding and failure mechanisms. We first show that the addition of nanofillers to hydrogels can greatly improve the material’s stiffness. We propose that this reinforcement stems from a local densification of the polymer around the nanofillers due to nanofiller-polymer attractive interactions. This densification enhances the stress coupling throughout the material, reduces the gel fluctuations, and ultimately leads to global stiffening of the composite. We then characterize the yielding and failure of the composite hydrogels. In these strain stiffening materials, we identify a transition to macroscopic irreversibility, beyond which the material cannot recover its full nonlinear viscoelastic response. This transition occurs at the strain at which the elastic modulus of the material is maximal. We reveal that the permanent damage is due to the breakage of bonds that are responsible for the material’s strain stiffening response. Finally, we relate the nonlinear mechanical response of two types of colloidal gels to the microscopic characteristics of their yielding, showing that colloidal gels of different interaction strengths exhibit distinct modes of failure. This work provides a comprehensive understanding of the behavior of multiphase soft materials under both small and large deformations. It establishes a framework that is critical to tailoring these materials for specific engineering applications and showcases the key effect of local mechanisms in the global mechanical response of materials.