Chemical Physics Seminar: Nanoscale Optical Imaging of Individual and Densely Packed Microgel Colloids

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​Abstract:

Microgels are among the most studied colloidal and polymeric systems of the past two decades. Swelling thermosensitive, poly(N- isopropylacrylamide) microgels by lowering the temperature provides a unique mechanism for controlling the porosity and size of colloidal particles on the nanoscale. As a consequence, these smart microgel particles are being considered for applications ranging from viscosity modifiers and sensing to drug delivery and as models for the glass and the jamming transition of soft colloids. Here, we present results from in-situ two-color superresolution microscopy of dye-labeled submicron-sized pNiPAM microgels [1,2]. We first demonstrate direct STochastic Optical Reconstruction Microscopy (dSTORM) to image single microgels in two and three dimensions, at different stages of the volume phase transition, with a lateral optical resolution of 30nm [3]. We find that the swelling behaviour observed in real space matches quantitatively with results from traditional light scattering measurements in reciprocal space. Next, we study dye labelled tracer microgels embedded in dense microgel suspensions. As we increase the packing density, we map out the different contributions that allow the dense packing of the soft microgels, due to deformation, interpenetration, and compression. Moreover, we can observe shape changes, such as faceting, in dense microgel systems where particles are compressed by the presence of its neighbours [4].  It is all but impossible to address these questions with other nanoscale imaging techniques used to date (AFM, Cryo-TEM) [2]. Based on the detailed understanding of the local structure and morphology we can model the macroscopic elastic properties of dense suspensions over a broad range of densities [5]. Our results suggest that due to lubrification mediated by the polymer brush-like corona, the friction between the microgels is reduced. Subsequently, with the onset of interpenetration, dissipation rises rapidly.