Aerodynamic focusing of nanoparticles: I. Guidelines for designing aerodynamic lenses for nanoparticles

This article describes the challenges in focusing nanoparticles (<30 nm) into tightly collimated beams, and provide guidelines for designing aerodynamic lens systems for nanoparticles. The major difficulties of focusing nanoparticles arise from their low inertia and high diffusivity. Because of their low inertia, nanoparticles tend to closely follow gas streamlines; their high diffusivities lead to beam broadening and diffusional deposition. We have identified the minimum particle size that can be focused to the axis with a single lens when diffusion is neglected, assuming that the flow is continuum and subsonic. We show that lighter carrier gases are preferred for focusing small particles, and that multiple lenses operating at sub-optimal Stokes numbers can be designed to focus particles smaller than was recognized previously. There exists a maximum pressure under which particles can be optimally focused, while particle diffusion and pumping requirements are minimized. Finally, we describe the procedure for designing aerodynamic lens systems for focusing nanoparticles, and present a case study of designing a single aerodynamic lens to focus 5 nm particles.