Understanding and improving aerosol focus in micro capillaries

The main goal of this research is to understand the behaviour of particles in a capillary. This knowledge is used to provide guidelines which will improve the focussing capabilities of an aerosol direct-write technique, namely the Aerosol Jet. In this study the particle radius ranges from 1 μm to 10 μm and the capillary radius varies from 50 μm to 400 μm. The first step to achieve this, is to develop an analytical model which describes the particle’s path. This model takes inertia, the Stokes and the Saffman forces into account. It is assumed that the carrier gas is incompressible. Particle-particle and mechanical particle-wall interactions are neglected. Secondly, the analytical model is used to gain understanding of the particle’s behaviour in a straight and converging capillary. This is done by case studies, to identify relationships between design parameters and focussing performance. Afterwards, the model is adjusted to study the integrated converging-diverging-converging (CDC) capillary, which is used in the Aerosol Jet. The effects of varying the particle density and radius, volumetric flow rate of the carrier, capillary radius and length are researched. Besides that, the influence of the particle’s initial radial position and velocity is also studied. It is found that the initial horizontal and vertical velocities in a converging capillary do not influence the particle’s path significantly, when the particle lags. However, the initial radial position does: when particles are released closer to the wall their focus is poorer. The range of volumetric flow rate that has been investigated has a minor influence on the focus. In addition, particles with a high radius and density have less tendency to follow the direction of the flow lines. This effect becomes larger when the particles are released far away from the capillary’s centre. Thirdly, the analytical model shows that the CDC capillary is not able to focus particles with a relatively high density and radius adequately. A new capillary with a converging-straight-converging (CSC) design has been studied. This CSC capillary has a much better focussing performance than the conventional CDC capillary. Still some small adaptations of the capillary radii may be required for different particle densities for the best focussing performance. It is also found that changing the lengths of the CDC parts while keeping the capillary radii constant does not result in significantly better focussing performance. By keeping the length of the first part the same and shortening the second and third part the focussing performance increases marginally. The lengths of the second and third sections need to be decreased as the particle density increases to improve the focusing performanceMechanical, Maritime and Materials EngineeringPrecision and Microsystems EngineeringMicor and Nano Engineerin