Macro- and Microrheology of Heterogeneous Microgel Packings

Microgels are soft deformable colloids that can be packed by external compression. Such packing transforms a suspension of loose microgel particles into an arrested state with properties similar to that of a macroscopic gel. This effect provides a way to purposely impart micrometer or submicrometer scale spatial inhomogeneities into these assemblies, allowing their effect to be studied. We follow this idea and prepare microgel packings that consist of major (50-99.7 No.%) fractions of soft, loosely cross-linked particles doped with defined minor (0.3-50 No.%) fractions of stiff, densely cross-linked particles. This approach creates soft microgel packings that contain defined submicrometer scale domains with very high degree of cross-linking, resembling the structure of inhomogeneous macroscopic gels. We study these inhomogeneous composites from macro- and microscopic perspectives by oscillatory shear rheology and fluorescence recovery after photobleaching to probe their macroscopic mechanics and the microscopic mobility of flexible linear tracer polymers that diffuse through them. These studies reveal an ambiguous behavior: whereas the presence of densely cross-linked domains does not exhibit any systematic effect on the bulk compressibility and microscopic tracer-chain diffusivity in the heterogeneous packings, it increases their macroscopic shear elastic modulus in a linear additive fashion. These results indicate that the impact of spatial inhomogeneities in polymer gels depends on whether the gels are probed in equilibrium or deformed states.