Assembly of Optical-Scale Dumbbells into Dense Photonic Crystals

Abstract

We describe the self-assembly of nonspherical particles into crystals with novel structure and optical properties combining a partial photonic band gap with birefringence that can be modulated by an external field or quenched by solvent evaporation. Specifically, we study symmetric optical-scale polymer dumbbells with an aspect ratio of 1.58. Hard particles with this geometry have been predicted to crystallize in equilibrium at high concentrations. However, unlike spherical particles, which readily crystallize in the bulk, previous experiments have shown that these dumbbells crystallize only under strong confinement. Here, we demonstrate the use of an external electric field to align and assemble the dumbbells to make a birefringent suspension with structural color. When the electric field is turned off, the dumbbells rapidly lose their orientational order and the color and birefringence quickly go away. In this way, dumbbells combine the structural color of photonic crystals with the field addressability of liquid crystals. In addition, we find that if the solvent is removed in the presence of an electric field, the particles self-assemble into a novel, dense crystalline packing hundreds of particles thick. Analysis of the crystal structure indicates that the dumbbells have a packing fraction of 0.7862, higher than the densest known packings of spheres and ellipsoids. We perform numerical experiments to more generally demonstrate the importance of controlling the orientation of anisotropic particles during a concentration quench to achieve long-range order.

Shaping colloids for self-assembly

“…Shape has a fundamental role in self-assembly as it can regulate particle recognition 3 , determine the density and the structure of particle packings [4][5][6][7][8] , and facilitate binding from ligand molecules 9,10 . Ideally, patterned surfaces and tailored functionality could act in concert with the shape anisotropy of the building blocks to focus interactions between particles, and could assist, direct or enhance their selforganization 11 .…”

Self-assembling knots of controlled topology by designing the geometry of patchy templates

“…For instance, colloidal scientists and soft matter physicists have explored the feasibility of self-assemblying extended photonic crystals and other metamaterials by designing patchy particles of increasingly complex surface activity [1][2][3][4] , by exploiting the shape anisotropy of convex polyhedrical colloids 5,6 or by choosing to disperse particles into complex fluids such as binary fluids or liquid crystals [7][8][9][10] . The resulting interparticle interactions are in general highly non-trivial and can promote the formation of crystals, glasses, gels and so on, often with tunable physical properties 11 .…”

Spontaneous shape reconfigurations in multicompartmental microcylinders

“…Thus, particle engineers have been accustomed to almost exclusively dealing with spherical polymer particles. Recent exceptions include the synthesis of certain inherently nonspherical polymer particles (9,(33)(34)(35)(36), energy-driven conversion of spherical particles into shaped particles via stretching at elevated temperatures (37)(38)(39), and the electrohydrodynamic cojetting procedure used herein to create microcylinders (30).…”