Making Soft Solids by Shear-assisted Assembly of Colloids – MS3AC

Numerous hard materials from major industries, i.e., foodstuff, personal care, and building materials, are obtained from soft viscoelastic precursors. In practice, controlling the properties of these soft precursors is critical to control the terminal properties of the corresponding hardened material. These precursors are Colloidal Gels (CGs), composed of subunits such as particles and polymers, whose attractive interaction and volume fraction control their macroscopic properties. More recently, it was shown that the microstructure and mechanical properties of CGs could be tailored through shear. Indeed, moderate shear triggers the growth of some structural anisotropy through the selective breakdown of the weakest bonds in the gel microstructure and shear-induced aggregation. Upon flow cessation, any structural anisotropy developed under shear is frozen in the CG microstructure, thus impacting its macroscopic mechanical properties. However, to date, a fundamental understanding of the subtle interplay between shear history, microstructural anisotropy, and shear-induced reinforcement of gels viscoelastic properties is still lacking. In this project, we will experimentally determine the physical principles that govern the shear-assisted assembly of CGs and the resulting impact on their microstructure and viscoelastic properties. We will use orthogonal superposition rheometry, microindentation, together with light scattering and low field NMR, to study and compare the microstructure and properties of two types of shear-sensitive systems, i.e., (i) self-aggregating gels that form spontaneously due to attractive interactions between their constituents and (ii) shear-induced gels formed by irreversible flocculation of colloids via polymer bridging. The ultimate goal and outcome of the current project are the design of time-dependent cessation flows to control the microstructure of CGs and make soft solids with on-demand macroscopic mechanical properties.