Effects of Solution Flow on the Growth of Colloidal Crystals

For the versatile potential applications of colloidal crystals, precisely controlling their growth is required to achieve properties such as high crystallinity and large-area crystals. Because colloidal crystallization is a self-assembly process of dispersed particles in a solution, solution flow directly and markedly changes the behavior of particles. Thus, the effects of solution flow on the growth of colloidal crystals were investigated in the present study. We found three different effects of solution flow on the growth of colloidal crystals: enlarging the first layer, facilitating the growth of superlattice structures, and forming a new circular packing structure. Specifically, in the single-component system, because the flow speed is lower closer to the bottom of the cell, the second and further layers dissolve owing to the large flow speed, whereas the first layer remains undissolved at the appropriate flow speed. The dissolved particles (particles that are detached from the crystals and returned back into the aqueous medium) are transported near the first layer, where they facilitate the growth of the first layer. In a binary system, when colloidal crystals with different particles are neighboring each other, the flow dissolves the surface of each crystal, which forms a dense, melt-like phase between crystals, from which a superlattice structure such as AB₂ grows. The flow often moves the second layer more than the first layer because the flow speed varies with the distance from the bottom. This causes the second layer to slide above the first layer of the neighboring crystals composed of different particle sizes, which transform from the initial face-centered cubic structure of the first layer into a circular pattern with strain. These findings contribute to establishing a sophisticated control method for growing colloidal crystals.