Focusing Particles with Diameters of 1 to 10 Microns into Beams at Atmospheric Pressure

We report on an ambient-pressure aerodynamic lens for particles in the 1-10 mu m diameter range, operating at a flow rate of about 1 lpm, that greatly increases the particle number density downstream of the lens. Intended for use with optical monitors, the lens's low pressure drop (10-30 Torr) minimizes pumping requirements and thus power consumption. Numerical modeling indicates that the orifice particle Stokes number and the ratio of orifice diameter to tube diameter are the critical determinants of lens performance. The gas Reynolds number (on the order of 1000, two orders of magnitude greater than typical lens systems used in aerosol mass spectrometers) has only a weak effect on both beam diameter and particle transmission efficiency. Model results, validated by light scattering experiments, indicate that a lens with a final tube with inner diameter 1.65 mm and a single orifice of 0.75 mm diameter can produce beam diameters of far less than 0.10 mm. The effective working distance of the system, similar to 1 cm downstream of the orifice, is limited by transition to turbulence in the jet downstream of the orifice. This also limits the lens design to a single orifice. Calculated and measured transmission efficiencies range from unity for particles smaller than 3 mu m to similar to 40% for 10 mu m particles. Simultaneous achievement of both a narrow beam and excellent transmission efficiency is limited to about a three-fold range in particle size, so no single lens of this design can simultaneously encompass the entire 1-10 mu m particle size design range.