Thomas Baron Award Lecture: Shear Thickening & Gelation in Colloidal Dispersions - Nonequilibrium States and Their Applications in Personal Protective Equipment

Colloidal gels and shear thickening fluids are challenging to formulate and process as they are often out of equilibrium states, but as will be shown, provide opportunities for engineering protective materials with a novel mechanical response. Developing fundamental structure property relationships for concentrated particle dispersions that exhibit shear thickening and/or gelation that derive from the properties of the particles that comprise the dispersion, their interparticle interactions, and their interactions with the suspending medium is scientific challenge with technological benefits. In this presentation I will discuss advances in measurement techniques that enable developing such relationships and demonstrate how this understanding can facilitate the development of novel personal protective equipment. Measurements of the microstructure commensurate with the viscosity and normal stress differences in shearing colloidal suspensions provide an understanding of how to control the viscosity, yield stress, shear thinning, and shear thickening rheological behavior. This is achieved through a combination of model system synthesis, rheological, and small angle neutron scattering (SANS) measurements under flow, as well as simulation and theory. Although many technological applications of concentrated suspensions are hindered by shear thickening behavior, novel field-responsive nanocomposites have been developed around shear thickening fluids (STFs). Ballistic, stab and impact resistant flexible composites are synthesized from colloidal & nanoparticle shear thickening fluids and gels for application as PPE. These rheological and microstructural investigations and micromechanical modeling serve as a framework for the rational design of STF-based materials to meet performance requirements.