Yolk/Shell Colloidal Crystals Incorporating Movable Cores with Their Motion Controlled by an External Electric Field

Abstract

Yolk/shell particles composed of a submicrometer-sized movable core and a silica shell are promising building blocks for novel optical colloidal crystals, because the locations of cores in the shell compartment can be reversibly changed by using external stimuli. Two dimensional arrays of yolk/shell particles incorporating movable cores were prepared by a self-assembly method. The movable cores of colloidal crystals in water could be observed with an optical microscope under application of external electric field. The motions of inner silica cores depended on the electric field strength and frequency and were categorized into three cases: (1) Random Brownian motion, (2) anisotropic motion of cores moving in a direction orthogonal to a field, and (3) suppressed motion fixed in the center of shell compartment. Random Brownian motion of cores was scarcely affected by field strength when a high frequency (in the MHz range) electric field was applied. On the other hand, an increase in field strength at low-frequency fields (kHz) transiently changed the core motion from (1) to (2) and a further increase in field strength changed it from (2) to (3). When the silica core was incorporated in a large void a stronger electric field was needed to suppress its motion than when it was in a small void. The high responsivity to electric fields in a low-frequency range indicated the importance of electric double layer (EDL) interaction between core and inner shell in controlling the core location in yolk/shell colloidal crystals. It was also shown that movable titania cores in yolk/shell particles required a low-frequency field with a high strength to change from the random to anisotropic motion. The result suggested that the electrostatic interaction between EDLs of the silica core and the inner silica wall could be stronger than that between EDLs of the titania core and the silica shell.

Auxeticity enhancement due to size polydispersity in fcc crystals of hard-core repulsive Yukawa particles

“…The electric field can also impact the optical properties of colloids composed from spherically symmetric particles. 12 In the case of spherical particles, a large difference in the elastic properties of face-centered-cubic (fcc) and hexagonal-closepacked hard-sphere crystals was also found. 13 The latter model molecule (hard sphere) is important for colloids and is the limiting case of Yukawa particles.…”

Chemistry and interactions of silica based particles studied by liquid cell electron microscopy

“…However, it is still safe to conclude that the ionic strength also if the presence of the particles was fully taken into account would increase. Because of the porous a b structure through which also the inner shell was removed [120] of the shell we assume that ions can pass through it so that the ionic strength inside and outside the shell was equalized. By screening the surface charge with ions, one can tune the Debye length and thus the repulsion-free region inside the shell.…”

“…As these particles consist of a core particle entrapped inside a rigid spherical shell, they are an ideal model system to experimentally study the diffusion of colloidal particles in spherical confined geometry and/or to be used in advanced e.g. photonic applications [120]. These rattle particles can be synthesized with a wide range of materials, particle sizes, shapes, and structures [54,55,69,120].…”

“…photonic applications [120]. These rattle particles can be synthesized with a wide range of materials, particle sizes, shapes, and structures [54,55,69,120]. Because of its excellent mechanical, chemical, thermal stability, transparency for visible light, and bio-compatibility, tunable porosity, silica is a promising material to produce rattle particles [74,120,160].…”

“…These rattle particles can be synthesized with a wide range of materials, particle sizes, shapes, and structures [54,55,69,120]. Because of its excellent mechanical, chemical, thermal stability, transparency for visible light, and bio-compatibility, tunable porosity, silica is a promising material to produce rattle particles [74,120,160]. Using silica as a negatively charged material in neutral pH conditions [161] to make the shell part of rattle particles, makes it necessary if the stability of the movable core is induced by charge interactions to also be negatively charged if the ionic strength is sufficiently low.…”