Structural Color of Colloidal Clusters as a Tool to Investigate Structure and Dynamics

Colloidal assemblies have applications as photonic crystals and templates for functional porous materials. While there has been significant progress in controlling colloidal assemblies into defined structures, their 3D order remains difficult to characterize. Simple, low-cost techniques are sought that characterize colloidal structures and assist optimization of process parameters. Here, structural color is presented to image the structure and dynamics of colloidal clusters prepared by a confined self-assembly process in emulsion droplets. It is shown that characteristic anisotropic structural color motifs such as circles, stripes, triangles, or bowties arise from the defined interior grain geometry of such colloidal clusters. The optical detection of these motifs reliably distinguishes icosahedral, decahedral, and face-centered cubic colloidal clusters and thus enables a simple yet precise characterization of their internal structure. In addition, the rotational motion and dynamics of such micrometer-scale clusters suspended in a liquid can be followed in real time via their anisotropic coloration. Finally, monitoring the evolution of structural color provides real-time information about the crystallization pathway within the confining emulsion droplet. Together, this work demonstrates that structural color is a simple and versatile tool to characterize the structure and dynamic properties of colloidal clusters.