Shape-sensitive crystallization in colloidal superball fluids

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 112; no. 17; pp. 5286 - 5290
• Main Authors: Rossi, Laura, Soni, Vishal, Ashton, Douglas J., Pine, David J., Philipse, Albert P., Chaikin, Paul M., Dijkstra, Marjolein, Sacanna, Stefano, Irvine, William T. M.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 28.04.2015; National Acad Sciences
• Subjects: Crystal lattices; Crystallization; Phase transitions; Physical Sciences; Silica; Simulation; superballs; dense packings; phase behavior; depletion interactions; Monte Carlo simulations
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1415467112

Guiding the self-assembly of materials by controlling the shape of the individual particle constituents is a powerful approach to material design. We show that colloidal silica superballs crystallize into canted phases in the presence of depletants. Some of these phases are consistent with the so-called “[Formula]” lattice that was recently predicted as the densest packing of superdisks. As the size of the depletant is reduced, however, we observe a transition to a square phase. The differences in these entropically stabilized phases result from an interplay between the size of the depletants and the fine structure of the superball shape. We find qualitative agreement of our experimental results both with a phase diagram computed on the basis of the volume accessible to the depletants and with simulations. By using a mixture of depletants, one of which is thermosensitive, we induce solid-to-solid phase transitions between square and canted structures. The use of depletant size to leverage fine features of the shape of particles in driving their self-assembly demonstrates a general and powerful mechanism for engineering novel materials. Significance Since antiquity it has been known that particle shape plays an essential role in the symmetry and structure of matter. A familiar example comes from dense packings, such as spheres arranged in a face-centered cubic lattice. For colloidal superballs, we observe the transition from hexagonal to rhombic crystals consistent with the densest packings. In addition, we see the existence of square structures promoted by the presence of depletion attractions in the colloidal system. By using a mixture of depletants, one of which is size tunable, we induce solid-to-solid phase transitions between these phases. Our results introduce a general scenario where particle building blocks are designed to assemble not only into their maximum density states, but also into depletion-tunable interaction-dependent structures.