(523i) Three-Dimensionally Ordered Assemblies of Yolk–Shell Particles with Controllable Regularity By Electric Field-Assisted Core Motion

Colloidal crystals, which are the ordered assemblies of colloidal particles, are known as a type of photonic crystals. Colloidal crystals composed of submicron-sized particles can reflect specific wavelengths in the range of the visible spectrum; therefore, they are promising candidates for optical devices. On the other hand, reconfiguration of the arrangement of component particles is generally slow and challenging once the crystal structure is formed. In this study, yolk–shell particles, which are a kind of core–shell type particles consisting of a hollow shell particle and a mobile core particle, were employed to develop novel colloidal crystals of which the assembled structure can be reversibly controlled.

Monodisperse yolk–shell particles composed of silica and titania were synthesized and then used as building blocks for the yolk–shell colloidal crystals. Because the core motion of yolk–shell particles can be controlled by applying external AC electric fields[1], the motion of the cores of the yolk–shell colloidal crystals was changed from random to coherently parallel to the field by applying an AC electric field. This change in core motion led to an improvement of the regularity of the cores’ arrangement; therefore, the Bragg reflection intensity from the yolk–shell colloidal crystals was quickly strengthened/weakened by switching an AC electric field on/off (frequency = 100 Hz)[2]. This switchability is applicable to a variety of fields such as sensors and displays.

The proposed yolk-shell colloidal crystals were, however, not durable enough for practical use; the arrangement of hollow shells could be collapsed by external stimuli like mechanical forces. To solve this problem, the assembled structure of colloidal particles was reinforced by a flexible hydrogel. The fabricated composite films of colloidal crystals and the hydrogel was free-standing; therefore, the three-dimensional structure was able to be observed via confocal microscopy. It was also confirmed that the core particles of the yolk–shell particles inside the hydrogel films exhibited random Brownian motion[3], suggesting that the hydrogel matrices are an appropriate material to immobilize the assembled structure of yolk–shell particles without sacrificing core mobility.

References:

[1] T. A. J. Welling et al., J. Colloid Interface Sci., 627, 761–773 (2022)

[2] H. Namigata et al., ACS Appl. Opt. Mater., 2, 181–190 (2024)

[3] H. Namigata et al., Colloids Surf. A, 690, 133781 (2024)