(128b) Ising Lattices of Asymmetric Colloidal Dimers Under Electric Fields

Studies on colloidal particles have shed light in the physics of soft materials, including the crystals nucleation and growth, phase behavior, and glass formation. In particular, anisotropic particles can potentially mimic natural interactions between molecules, which are often shape-specific and orientation-dependent. Arguably, the simplest form of anisotropic particles beyond spheres is colloidal dimer where two lobes are grown or fused together. Here we report the directed-assembly of geometrically anisotropic colloidal dimers under the influence of AC electric fields. By combining the anisotropy in both particle geometry and field-induced electrostatic/hydrodynamic interactions, we show a variety of novel structures that closely resemble one- and two-dimensional Ising lattices, including small clusters, linear chains, square crystals, and frustrated triangular arrays. All of them are uniquely formed by alternating association between dimers with opposite orientations. To understand the underlying physics of the structural formation, we develop a theoretical model based on the energetic approach. Our model reveals that the overall pair interactions between asymmetric dimers depend sensitively on their relative orientations. Moreover, the potential between oppositely oriented dimers consists of a concave shoulder with two different length scales, which well explains the unique structures that we observe in experiments.