Field-induced tubular assembly of ionic microgels: Evidence for shape anisotropy from small-angle neutron scattering...

We report the AC electric field-induced directed assembly of 10 mol% crosslinked ionic microgels at various effective volume fractions, investigated using confocal microscopy and small-angle neutron scattering. At low effective volume fractions, in an electric field, the microgels form strings, tubes, and islands of body-centered-tetragonal structures. While tubular structures have previously been reported for intrinsically anisotropic particles such as ellipsoids or bowl-shaped colloids, analogous assemblies have not been observed for initially isotropic hard or soft particles. Small-angle neutron scattering experiments under the so-called zero-average contrast conditions were carried out to obtain the single-particle size and structure as the microgels align along the direction of the field. The P(r) analysis of the small-angle neutron scattering data reveals a clear signature of elongated ellipsoidal particles. A fuzzy-ellipsoid model was thus employed to fit the data, yielding aspect ratios ranging from 1.93 to 2.46 depending on the field strength and effective volume fraction. We therefore propose that the tubular assemblies observed at low effective volume fractions arise from field-induced shape anisotropy of the microgels. Our results demonstrate how external electric fields can dynamically induce particle anisotropy in initially spherical soft colloids, thereby enabling the formation of new self-assembled structures that are typically associated with anisotropic building blocks. We discuss the experimental phase diagram and compare our observations to the theoretical phase diagram for soft dipolar spheres.