Rheological properties and microstuctural analysis of bimodal suspensions with highly size-asymmetric particles

Abstract

Coating fluids that include highly size-asymmetric particles are widely used in many applications such as Li-ion batteries, multi-layer ceramic capacitors, and electrical conductors. They include different types of particles to improve the performance of the product. In these liquids, the size of the particles is very different

one is micrometer and the other nanometer scale, for example. Because of distinctively different size of the particles, these bimodal dispersions show more complex flow behavior compared to colloids of a single size. However, most studies for bi-modal dispersions assumed well-stabilized nanoparticles, thus they have not been relevant to the behavior of the materials used in industry. In this study, a model system consisting of two kinds of particles with highly asymmetric size was designed to offer a valuable description of complex behavior of the slurries, using a suspension consisting of polystyrene latex (500 nm) and alumina-coated silica (12 nm) particles. The surface potential of small particles was tuned by varying the solution pH, causing them to be repulsive to each other, attractive to each other, and oppositely charged to the large particles, while the large particles remained electrostatically stabilized. The effect of addition of small particles on the rheological properties of bimodal suspensions was investigated in terms of surface chemistry and concentration of small particles. The rheological properties were dramatically changed from viscous to gel-like depending on the surface potential and concentration of small particles. A colloidal gel was induced by small particles when the small particles had the opposite charge to the large particles and a volume fraction of , and when the small particles were attractive to each other above a critical threshold, . Cryo-SEM distinguished the gel structures to be either short bridging gels produced by oppositely charged small particles, or long bridging gels or dense gels produced by attractive small particles. On the basis of this rheological behavior and microstructure, a phase diagram of highly size-asymmetric bimodal colloids was presented with respect to the surface chemistry and concentration of small particles. When the mutually attractive small particles are added to suspension of highly charged large particles, a new type of colloidal gel was induced which is not described by the typical power - law scaling for fractal clusters. Their elastic moduli have a unique scaling behavior on particle volume fraction with two distinct power - law indices. The unique scaling behavior arises from the non-fractal networks of large particles that are bridged by small particle clusters in the region between the lower and the upper critical boundary of small particle volume fraction. Because the model fluid in this study is similar to the slurries used in industry with respect to the size ratio, the range of concentration, and the surface potential, it is expected that the slurries encountered in practice have analogous mechanical behavior and microstructure to this model fluid. This study consequently provides a guideline for the design of such complex fluids and understanding of their complex flow behavior.