Superresolution Microscopy of Individual and Densely Packed pNIPAM Microgels

Microgels made from poly(N-isoprolylacrylamide) (pNIPAM) change their conformation in response to changes in temperature or solvent composition. Below ~32°C they are swollen and have a core-shell structure, while above they collapse and are akin to hard spheres. Their core-shell structure allows them to be packed far beyond jamming. An exhaustive description of dense suspensions relies on independent characterization of individual microgels. However, non-isotropic shape deformations are hardly accessible via scattering techniques and single particle observations in situ have so far been hindered by insufficient resolution, with optical microscopy, or contrast, with cryo-TEM.
Recently developed super resolution microscopy techniques can provide nanoscopic optical resolution. We demonstrate their validity by imaging single microgels at different stages of the volume phase transition. Next, we map out the different contributions of deformation, interpenetration and compression from the marginally jammed state to deeply into the overpacking regime. We find that initially interpenetration and shape deformation are dominant until, in the highly overpacked state, only compression remains as a means for further densification of the suspension.