Controlling the deposition resolution of nanoparticle aerosols using aerodynamic focusing

Abstract

The demand for nanoparticles in various applications is increasing. These applications can only be realised if nanoparticles can be specifically arranged and patterned on a substrate. This thesis considers a direct write method that uses an aerodynamic focusing nozzle to deposit these particles. A major challenge arises when the nanoparticles are decreased in size. This research studies the control of the deposition resolution of small sized nanoparticles (≤10 nm) in an aerosol flow. A FEM model was developed to describe the particle’s path. These models were made for a converging and converging sheath gas nozzle. Due to the small particle sizes considered, it was found that only the Stokes (drag) force needs to be accounted for during modelling. The effect of different nozzle exit throat, working distance, angle and flow rate configurations are studied. Nozzle designs were evaluated using three performance criteria, namely the contraction factor, focusing ratio and line width. These describe the contraction of the particles within the nozzle system, the focusing after the nozzle system and the width of the line, respectively.
It was found that smaller angles, longer converging sections and higher velocities resulted in smaller line widths. Also, the contraction factor hardly depends on the particle size. Smaller nozzle exit throats have significantly higher focusing ratios for particles smaller than 10 nm. The working distance does not effect the contraction factor directly, indicating capabilities of deposition on non-flat surfaces. The converging nozzles can only deposit ≤10 nm particles if the nozzle exits are sufficiently small. This can cause clogging. The focusing ratios, in these nozzles, never exceeds the value of one, indicating always a larger line width than the nozzle exit diameter.
In the sheath gas nozzle system, high sheath gas ratios are essential for increasing the contraction factor and particle velocity. This prevents clogging. Introducing the sheath gas earlier in the nozzle system is more effective than at the end. The best modelled contraction factor, focusing ratio and line width achieved, using 10 nm particles, with a converging nozzle are 1.0, 0.4 and 874 microns, respectively. This nozzle has a nozzle exit throat of 400 microns and an angle of 10 degrees. However, a modelled contraction factor of 9.3, focusing ratio of 3.8 and line width of 104 microns are achieved using a converging sheath gas nozzle with a nozzle throat of 400 microns and an angle of 5 degrees. Narrower line widths are expected if the throat diameters are reduced.
For the experiments, a setup was used in which nanoparticles are generated using a spark ablation method. During these experiments it was seen that the maximum flow rate is mainly dependent on the nozzle exit throat of the aerosol channel. The line widths, during experimenting, were found to be wider due to inaccuracies in the stage and the differences in settings between the model and setup.