Transparent Colloidal Crystals With Structural Colours

Yang Lan,Talha Erdem,Peicheng Xu,Alessio Caciagli,Mykolas Zupkauskas,Erika Eiser,Peter Bösecke,Thomas O’Neill

doi:10.3389/fphy.2022.847142

Abstract

Spatially ordered arrangements of spherical colloids are known to exhibit structural colours. The intensity and brilliance of these structural colours typically improve with colloidal monodispersity, low concentrations of point and line defects and with increasing refractive index contrast between the colloids and the embedding medium. Here we show that suspensions of charge stabilised, fluorinated latex particles with low refractive-index contrast to their aqueous background form Wigner crystals with FCC symmetry for volume fractions between 13 and 40%. In reflection they exhibit both strong, almost angle-independent structural colours and sharp, more brilliant Bragg peaks despite the particle polydispersity and bimodal distribution. Simultaneously, these suspensions appear transparent in transmission. Furthermore, binary AB, A2B and A13B type mixtures of these fluorinated and similarly sized polystyrene particles appeared predominantly white but with clear Bragg peaks indicating a CsCl-like BCC structure and more complex crystals. We characterised the suspensions using a combination of reflectivity measurements and small-angle x-ray scattering, complemented by reflectivity modelling.

Highlights

Colloidal particles are ubiquitous: they are a major ingredient in cosmetics [1], agriculture [2], bioimaging [3] and electronics [4], to mention but a few
The reflection spectra show strong, sharp, and clear Bragg peaks despite the particle polydispersity and bimodal distribution. We found that these fluorinated latex (FL) particles form Wigner crystals with face-centred cubic (FCC) symmetry for volume fractions between 13 and 40% leading to a concentrationdependent iridescent appearance
The group of Asher [45] refined the synthesis of these fluorinated latex particles using emulsion polymerization of 1H,1H-heptafluorobutyl methacrylate monomers to obtain highly charged, monodisperse colloids ranging in size from 50 to 250 nm

Summary

Introduction

Colloidal particles are ubiquitous: they are a major ingredient in cosmetics [1], agriculture [2], bioimaging [3] and electronics [4], to mention but a few. Due to their size, ranging from tens of nanometres to a few micrometres, they are known to display structural colours when assembled into crystals [3,5–10]. For this reason, a lot of research effort has been devoted to the study of colloidal crystallisation [11–17]. Use of long polymers grafted to nanoparticles [37] is another means to introduce ‘soft’ interactions

Methods

Results

Discussion

Conclusion