How Do Colloidal Gels Form and Flow?

Norm Wagner from Univ of Delaware

Colloidal gels are among the most prevalent man-made and natural materials, ranging from pastes for solar cells, to cement, foods and biocolloids.  Such systems are also rheologically complex, exhibiting aging and thixotropy. We study a model, thermoreversible nanoparticle gel at equilibrium and under steady and time-dependent shear flow with the goals of experimentally developing a universal state diagram for the adhesive hard sphere potential, testing prevailing theory and simulation, and providing a basis for understanding concentrated protein solutions.  A model thermoreversible adhesive hard-sphere system composed of octadecyl-coated silica particles suspended in n-tetradecane is synthesized and fully characterized. Temperature variation enables reversibly transitioning the system from behaving as a near hard-sphere to an adhesive hard-sphere system leading to aggregation and ultimately, a dynamical arrest transition to macroscopic gelation (Langmuir 28, 1866−1878 (2012)). A universal state diagram is presented for the adhesive hard sphere model system including dynamical transitions of gel and attractive driven glass formation (PRL 106, 105704 (2011)).  Complementary simulations demonstrate that these transitions are the result of rigidity percolation, first proposed by He and Thorpe in 1985, thereby connecting dynamical arrest in the AHS system to the more general phenomenon of dynamical arrest in network forming materials (PRE 88, 060302(R) (2013)).  We also resolve whether phase separation is a prerequisite for gel formation and provide quantitative predictions of gel stability in an external field, such as gravity, and identify the limits of stability and the role of spinodal decomposition in gel formation (PRL 110, 208302 (2013)). Novel neutron scattering measurements under dynamic shear flow help determine the mechanism by which gels flow and the effects of flow on gel structure (JOR, 58, 1301 (2014)). A universal behavior is discovered for the shear viscosity and microstructural evolution under steady shear that provides guidance for understanding gel rheology and for processing gels to achieve a desired microstructure.  Finally, the recent application of these methods to the study of protein gels and concentrated monoclonal antibody solutions, will be discussed (Biophys. J., 106, 1763(2014), PRL, 115, 228302(5) (2015)).