Fast formation of opal-like columnar colloidal crystals

Area of Science:

• Materials Science
• Colloid Science
• Crystallography

Background:

• Colloidal crystals, such as opals, exhibit iridescent colors due to Bragg reflections.
• Controlling the self-assembly of polydisperse colloidal particles into ordered structures remains a challenge.

Purpose of the Study:

• To investigate the self-organization of highly polydisperse colloidal gibbsite platelets into opal-like structures.
• To explore the effect of centrifugation on the formation process and resulting crystal properties.

Main Methods:

• Transmission electron microscopy (TEM) for visualizing particle arrangement.
• Small-angle X-ray scattering (SAXS) for analyzing structural ordering.
• Centrifugation at 900 g to accelerate crystal formation.

Main Results:

• Highly polydisperse gibbsite platelets readily formed columnar opal-like crystals with iridescent Bragg reflections.
• Centrifugation significantly accelerated crystal formation without inducing a disordered glassy state.
• Forced sedimentation led to particle size fractionation and an inversion of iridescent colors.
• Evidence of hexatic-like ordering was observed, potentially explaining the self-organization.

Conclusions:

• Centrifugation is an effective method to rapidly form ordered colloidal crystals from polydisperse systems.
• Particle fractionation during centrifugation plays a key role in the self-organization and color properties of gibbsite crystals.
• The findings suggest a pathway for designing iridescent materials from polydisperse colloidal building blocks.