Mapping mesoscale heterogeneity reveals the effect of structure and preparation protocol on the rheology of microgel suspensions

The bulk rheology of microgel suspensions and other soft materials can be severely affected by the size and degree of clustering of their building blocks, which often span multiple length scales and can sensitively depend on preparation protocols. To evaluate these effects, microrheology techniques based on optical microscopy can provide critical information. However, they typically require a resolution which limits the field of view and thus hampers the access to large-scale heterogeneities. Here, we propose a simple and efficient approach to map rheological properties up to the millimeter scales, based on low-magnification microscopy and tracking of the motion of dispersed tracers under weak pressure gradients generated by capillarity effects in confined samples. The time scaling of the mean square displacement of particles allows us to identify the prevailing regime-arrested, diffusive, or ballistic-in each sample region, generating for the first time heterogeneity maps over a large field of view and providing insights into the mesoscopic structure. We apply this method to various types of microgel suspensions, undergoing different preparation protocols and displaying different bulk rheological properties. Our analysis allows to extract a spatial map of clusters in the samples and their size distribution, which result from the overall applied shear during preparation and in turn determine bulk microgel viscosity.