Deformable particles with anisotropic interactions: unusual field-induced structural transitions in ultrasoft ionic microgel colloids

Ionic microgels are intriguing soft and deformable colloids with an effective pair potential that crosses over from Yukawa-like at large distances to a much softer repulsive interaction at short distances. Here we report the effect of adding an anisotropic dipolar contribution to colloids with such ultra-soft interactions. We use an alternating electric field to induce a tunable dipolar contribution, and study the resulting particle self-assembly and phase transitions in situ with confocal laser scanning microscopy. We find significant field-induced structural transitions at low as well as at very high effective volume fractions. At phi(eff) = 0.1 we observe a transition from an isotropic to a string fluid. At phi(eff) = 0.85, there is a reversible transition from an amorphous to a dipolar crystalline state, followed by the onset of a gas-(string) solid coexistence. At phi(eff) = 1.6 and 2.0, i.e. far above close packing, evidence for a field-induced arrested phase separation is found.