Effect of Particle Stiffness On Glassy Dynamics of Dense Colloidal Liquids in the Bulk and Confined Regimes

“Fragile” glassy materials show a non-Arrhenius dependence of relaxation time with temperature close to the glass transition and have been extensively studied for molecular glass formers as model “hard-sphere” colloidal suspensions, but we lack a complete understanding of "strong" glass formers which show an Arrhenius dependence on temperature approaching the glass transition. In this work, we investigate the glassy dynamics of microgels of varied particle stiffness in dense aqueous suspensions using confocal microscopy. Poly(N-isopropylacrylamide) (PNIPAM) microgel particles of variable stiffness in aqueous media are synthesized by free radical polymerization varying the cross-linking density. We investigate five separate crosslinking densities to fully encompass the “soft” to “stiff” transition.  Our results show that “soft” particles and “stiff” particles behave as ideal strong glass formers and fragile glass formers, respectively. By exploring the intermediate crosslinking densities, we offer insight into the transition from a strong glass former to a fragile glass former, specifically that dynamic heterogeneity, specifically string-like motion, is more pronounced as stiffness increases. Furthermore, when these glasses are confined to a thickness of several particle diameters, these suspensions show arrested particle motion at smaller volume fractions while the dynamic heterogeneities show notable increase.