Application of a linear electrodynamic quadrupole trap for production of nanoparticle aggregates from drying microdroplets of colloidal suspension

We have made advancements in a linear electrodynamic quadrupole trap (LEQT) to produce various nanoparticle microaggregates and drop patterns by evaporation-driven aggregation in levitating droplets of colloidal suspensions and to analyse their properties. The LEQT is mounted in a small chamber with well controlled atmospheric parameters, such as temperature and relative humidity, to trap droplets ranging from similar to 70 mu m down to similar to 1 mu m in diameter and provide a controlled environment to study formation of nanoparticle aggregates. Transient and final dry microstructures formed during the droplet drying life-time have been deposited on a silicon substrate placed in the bottom part of the trap and studied off-line with scanning electron microscopy (SEM). The experimental results are supported/supplemented by the numerical model of evaporation-driven aggregation under various conditions. The application of the CUDA technology allowed us to simulate aggregation of a few hundred thousand nanoparticles of several types. Using the LEQT we investigated highly-ordered aggregates of silica nanoparticles, composites of silica and gold nanoparticles as well as sodium dodecyl sulfate (SDS) crystalline microstructures and aggregates of silica nanoparticles with SDS. We observed the formation of microstructures at the successive stages of aggregation and under various experimental conditions. The observed final microstructures demonstrate the technical capabilities and versatility of the LEQT for its adaptation for droplet drying scale-up applications such as spray drying.