Complex rheology of silica and protein colloids

The field of colloid science has seen a marked increase in attention in recent years, as suspended particulates may display unique physical interactions. The large surface area and magnetic/dielectric properties of silica particles in suspension result in different behavior than in its constituent atoms or bulk lattice structures. Silica suspensions are being considered for a protein-drug conjugate delivery system. Understanding the microscopic structures of the colloid under mechanical forces is essential, in order to predict macroscopic rheological conditions such as flow velocity, vorticity, and viscosity. Systems with 0.2 mass fraction silica particulate with varied mass fraction of lysozyme and BSA from 0.005 to 0.05 were tested in a rheometer, using both cup and bob and parallel plate geometries. Experimental results from Wang et al. (2018) were successfully reproduced with the lysozyme systems with 12 nm size silica. Lysozyme systems demonstrated shear thinning behavior as a result of the electrostatic interaction between silica and the protein, changing the net electrostatics of the conjugate from repulsive to attractive. There is preliminary evidence for anisotropy and the locality of a transient elastic zone (TEZ) of the conjugate system, as reported by Sanchez-Diaz et al. (2019). The storage and loss moduli invert dominance within a predicted range, demonstrating a shift from elastic solid behavior to fluid behavior over greater distances.