Direct visualization of strain-induced yielding in colloidal gels

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

0853648 M. Solomon 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, coatings and consumer products. 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. A unifying feature of these technologies is their dependence on the fact that gels yield if a stress or strain of sufficient magnitude is applied. This yielding is a poorly understood convolution of colloid pair interactions and gel microstructure. Yielding has features common with mechanical failure: a dramatic rheological transition results in fluidization of the previously rigid material. Recently, substantial progress has been made in both experimental description and theoretical explanation of the origin of gelation. However, from the point of view of engineering design and practice we require more: we must also understand stress-induced yielding, rupture and fluidization. Whether our interest is to produce a microfluidic valve that will open at a critical stress, or a detergent that will remain homogeneous and stable over its product life, we should address: What is the sequence of events that leads to gel rupture and internal failure upon application of strain and how does manipulating gel structure affect this sequence? How do these transitions feedback into suspension microdynamics to determine the local yield rate? How does an applied strain induce evolution of the stress bearing backbone of a gel network? To address these scientific questions, we will execute a research program to directly visualize strain-induced yielding and internal failure in gels. The intellectual merit of our research plan arises from our comprehensive application of confocal optical microscopy in pursuit of these aims and our development of novel, well posed methods to induce yielding in colloidal gels and study its implications. First, 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. Since yielding is a local phenomena, the direct visualization methodology is a key strength of our approach. Second, we recognize that previous attempts to visualize internal failure and rupture of gels have foundered because the nonideality of shear banding was encountered. Because shear banding is particular to the flow geometry studied, it does not directly characterize yielding, an intrinsic material property of broad fundamental interest. To address this issue, we will directly visualize yielding by high-rate stepstrain deformation. The literature and our prior work demonstrate that this flow avoids shear banding by generating homogeneous yielding and rupture of colloidal gels. In this project, we seek to extend fundamental understanding to the microscopic scale by probing the step-strain induced rupture of gels comprised of micron-scale sterically-stabilized colloidal poly(methyl methacrylate) in refractive-index and density-matched solvents. Because this system's pair potential interactions are both tunable and well characterized, results for this model system are applicable to the broad range of materials and gel structures encountered in engineering practice. Three tasks will be executed to address the three fundamental questions posed above. Project outcomes will include the first experimental assessment of the local yield rate of a colloidal gel, a key input to the successful soft glassy and model coupling models of gel rheology. This study will broadly impact technology and engineering in diverse areas such as ceramic, membranes, consumer products and direct write assembly by discovering fundamental features of the relationship between gel yielding and microstructure. Additional outcomes with broader impact include: (i) the training of a graduate student in state-of-the-art methods in confocal microscopy, colloidal science and rheology; (ii) development of a new engineering design component for a summer outreach program that introduces middle school girls to chemical engineering and materials science through hands on lab activities and experiments in complex fluids.