Future Directions in Nano-Scale Systems, Dynamics and Control

A report on the 2003 NSF workshop on nano-scale systems: "After giving a general background on nano-scale dynamics and systems, research problems and open issues related to Scanning Probe Microscopy dynamics and controls, nanomanipulation systems, directed self-assembly, nano-manufacturing, micro/nano-robotics, nanoelectromechanical sensors and devices, micro/nano-electromechanical systems integrated with biological entities, and nano-scale human-machine interfacing are discusse

Generating Particle Beams of Controlled Dimensions and Divergence: II. Experimental Evaluation of Particle Motion in Aerodynamic Lenses and Nozzle Expansions

A particle-beam-forming apparatus for producing narrow particle beams was developed based on the theory discussed in paper I of this series. It consists of a variable number of aerodynamic lenses (short capillaries and/or thin-plate orifices with diameters ranging from 3.5 to 7.0 mm) followed by an accelerating nozzle (3 mm). It was evaluated using monodisperse DOS and NaCl particles (0.02-0.24 μm) at upstream pressures on the order of 1 torr. The particle beams produced by the lens-nozzle system were focused through a skimmer (1 mm) into a high vacuum chamber (10-4-10-5 torr) where the beam widths, velocities and transport efficiencies were measured. The experiments showed that as more lenses were added the particle beam widths were reduced asymptotically to the minimum values. For spherical particles (DOS) these minimum values are in good agreement with the Brownian limit derived in paper I. For nonspherical particles (NaCl) these minimum widths are much larger than the Brownian limit, indicating that beam broadening is dominated by lift forces (see paper I). The particle transport efficiencies through the lens-nozzle-skimmer system exceed 90% for particle sizes from 0.03 to 0.24 μm. The measured beam velocities are also in good agreement with the calculated values

Generating Particle Beams of Controlled Dimensions and Divergence: I. Theory of Particle Motion in Aerodynamic Lenses and Nozzle Expansions

A particle beam is produced when a particle-laden gas expands through a nozzle into a vacuum. This work discusses the theoretical basis of a novel method for producing highly collimated and tightly focused particle beams. The approach is to pass the particle-laden gas through a series of axisymmetric contractions and enlargements (so-called aerodynamic lenses) before the nozzle expansion. Particles are moved closer to the axis by a lens if the particle sizes are less than a critical value and particles can be confined very closely to the axis by using multiple lenses in series. Since particles close to the axis experience small radial drag forces, they stay close to the axis during nozzle expansion and therefore form a narrow particle beam downstream. The major effects that limit the minimum beam width are Brownian motion and lift forces on particles during the nozzle expansion. Simple theoretical models are developed in this work to estimate the minimum particle beam width set by these effects. While the Brownian-motion effects occur for all types of particles, the lift-force effects only occur for nonspherical particles but are often much greater than the Brownian-motion effects