Gels of colloidal particles are systems with slow, constrained dynamics and unusual, viscoelastic rheology. They are central to the chemical processing of ceramics, the formation of membranes for microfiltration and the quality of paints, finishes and coatings. Next generation technologies such as direct-write assembly and microfluidic valving also rely on the gelation transition and the rheological properties of colloidal particle gels. Thus, fundamental understanding that could be applied to predict the gelation transition, to control the microscopic structure and dynamics of gels as well as to exploit their unusual non-linear rheology would broadly impact technology development in these areas. Scientifically, there is a need for experiments that can discriminate between theories of the gelation transition that are based on the mode-coupling framework and on the dynamic equilibrium clustering approach. In addition, the relationship between structural heterogeneity and gel microhydrodynamics should be discovered to understand the role of dynamic heterogeneity in gels. Finally, even qualitative features of local structural evolution upon the application of non-linear deformation have yet to be resolved through experiment. To address the scientific questions that underlie these technological needs, we will execute an experimental research program to observe local correlations among structure, dynamics and non-linear viscoelasticity of colloidal gels.
The intellectual merit of our research plan arises from our comprehensive application of confocal optical microscopy in pursuit of these aims. The power of confocal microscopy rests on its ability to directly visualize local, colloid-level structure and dynamics in three dimensions (3D) and with nanoscale resolution. We will principally study suspensions of micron-scale stericallystabilized colloidal poly(methyl methacrylate) in refractive-index and density-matched solvents. Short-range attractive interactions leading to gelation will be induced by non-adsorbing polymer. The control parameter governing the strength of the short-range attraction will be the concentration of polymer. Confocal microscopy of 3D image volumes will be used to quantify the size and heterogeneity of clusters and strings induced by gelation. The local structure will be characterized by measurement of the distribution of contact numbers of particles in the gel. These structural measures will be used to test specific predictions of mode coupling and thermodynamic theories of gelation. To study dynamical heterogeneity and other microhydrodynamical features of gels, structure will be correlated with single and collective particle dynamics quantified by 3D particle tracking. Finally, transient structural evolution will be monitored in start-up of steady-shear flow and after step-strain experiments through in situ confocal microcopy. Novel aspects of the flow experiments will be their attention to non-linear phenomena, their control of wall slip through surface topology engineering, and their execution with materials that will yield a never before available picture of the local, rotational dynamics of colloids in the gel. These direct visualization experiments are distinct from previous light, neutron and X-ray scattering studies because they specifically probe local phenomena and distributions of structure and dynamics that cannot commonly be obtained from the ensemble-averaged results of scattering. This study will broadly impact technology and engineering in areas as diverse as ceramic, membranes and direct write assembly by its elucidation of new scientific understanding of the relationship among the gelation transition, microscopic gel structure and dynamics as well as macroscopic flow. Additional outcomes with broader impact include the training of one graduate student in state-of-the-art methods in confocal microscopy, colloidal science and rheology as well as new development of a summer outreach program that introduces middle school girls to science and engineering through focused, hands on lab activities and experiments in complex fluids, chemical engineering and materials science.
