The influences of jet axis switching and aerodynamic focusing on aerosol deposition in converging–diverging slit impactors

Abstract

The deposition pattern or “linewidth” of an inertial particle deposit from acceleration of an aerosol through a high aspect ratio slit nozzle-substrate system is influenced by particle size-dependent aerodynamic focusing. In addition to this, high aspect ratio jets will eventually undergo downstream “jet-axis switching”, wherein the jet profile shrinks along its major axis and elongates in the direction of the minor axis. Jet axis switching in an aerosol may also lead to “rotated” particle deposits. In this study, we use a converging–diverging slit nozzle system with a major axis length of 8 mm and a throat width (minor axis) of 200 $\mu$m to examine the deposition patterns of monodisperse particles in the 100 nm–5 $\mu$m diameter range, in air with an upstream pressure of 252 Torr and variable downstream pressure in the 3–50 Torr range. The nozzle-to-substrate distance $L^{\ast }$ is varied from 90 to 248 times longer than the throat width. We find deposition patterns that are strongly dependent on particle size, downstream pressure, and $L^{\ast }$. Depending on particle diameter and operating conditions, we obtain deposits resembling the nozzle dimensions and orientation, deposits which are completely switched (perpendicular to the nozzle), or deposits which are relatively symmetric and focused at a center point. The latter appear to be the result of the combined effects of jet axis switching and aerodynamic focusing. In general, the influence of jet axis switching is more pronounced on smaller particles at higher downstream pressures, and with larger distances to the substrate. The extent of switching and area of the deposit are also both inversely related to the particle Stokes number.