Rattle-type particles: Tuning colloidal interactions and electrokinetics for switchable colloidal crystals

Abstract

‘Rattle-type’ particles consist of a moveable particle inside a spherical hollow shell. The shells are smaller than a micrometer and the particles within are even smaller. When such shells are filled with liquid, the particles move around due to collisions with molecules in the liquid. These rattle-type particles are interesting for multiple applications, such as switchable optical devices. In this thesis we investigated how we can control the movement of the particles for such applications. We did this using confocal microscopy, a version of high resolution light microscopy, and a novel technique called liquid-phase electron microscopy (LPEM). LPEM is often limited by the influence of the electron beam on the sample. In this thesis we take extra care to avoid such effects. In the study of rattle-type particles we started by looking at the diffusion of the core particles within their respective shells in 3D. The particles were observed to slow down considerably near the shell wall. By changing the amount of salt in our liquid, the spherical particles could be tuned to be confined to the middle of the shell, or rather move throughout the whole shell geometry. Significant double-layer overlap was observed in aqueous solution without added salt. Additionally, electric fields of varying frequency could be used to control the position and direction of the movement of the particle within its shell. At low frequencies the particles move parallel to the electric field, while at higher frequencies they move orthogonally. Next, we put many of the rattle-type particles together in an ordered structure by self-assembly. The shells could not move, but we still had control over the movement of the particles within. By using low-frequency electric fields we could manipulate the particles to also form ordered structures. This resulted in optical effects that can be used for future switchable optical devices and/or displays.