The role of ions in the self-healing behavior of soft particle suspensions

Understanding when a material crystallizes is of fundamental importance in condensed matter. In many materials, the presence of point defects suppresses crystallization. Surprisingly, colloidal hydrogels can overcome this limitation: A small number of large microgels can spontaneously deswell to fit in the crystal lattice of smaller microgels, thus avoiding the occurrence of point defects. We find that this unique particle deswelling is due to an osmotic pressure difference between the inside and the outside of the microgels resulting from the overlap of counterion clouds of neighboring particles. When this pressure difference exceeds the bulk modulus of the large microgels, these shrink, enabling crystallization without point defects.