Phase behavior in polydisperse microgel suspensions controlled by spontaneous particle deswelling

Crystallization is often suppressed by point defects due to larger impurity particles. Surprisingly, microgels can overcome this limitation: Large microgels can spontaneously deswell to fit into the crystal lattice of smaller but otherwise identical microgels. We find this particle deswelling to be triggered by a difference in osmotic pressure, Π, between the inside and the outside of the particles. Π is set by the counterions of charged groups on the microgels. A Π-difference between inside and outside of a microgel builds up when the counterion clouds of neighboring particles overlap. This causes an increase of Π in the space between particles, which is not compensated inside the particles. With increasing concentration, this Π-difference exceeds the bulk modulus of the softest and largest microgels and makes them deswell, enabling crystallization. We find the freezing point of poly- and bidisperse suspensions to be linked to particle deswelling: A reduction of polydispersity due to deswelling is required for crystallization. Compared to monodisperse suspensions, this causes the freezing point to shift to higher concentrations. In comparison to incompressible colloids, this particle deswelling mechanism fundamentally changes the role of polydispersity in microgel suspensions.