Fig. 1. A sketch of the aerodynamic separation of particles in a gas flow according to their inertia. For large particles St  1 i.e. their inertia largely exceeds the drag action of the gas flow. At a point where the flow-lines curvature is sufficiently high, the particle trajectories separate from the flow-lines and the particles are eventually deposited on the walls. Very small particles closely follow the flow-lines because their inertia being negligible compared to drag (St  1). Particles of some intermediate size have trajectories only slightly decoupled from the flow-lines and this can be exploited to concentrate particles at given positions in the flow-field.

Fig. 2. A schematic view of the aerodynamic focusing device introduced by Piseri et al. [34]. The 3D picture represents a sectioned focuser-cylindrical nozzle assembly. The aerosol moves from the left (large cylindrical section) to the right, passing through the holes on the periphery of the cylinder; then it flows radially towards the entrance of the narrow cylindrical section, where a sudden turn of the flow-lines produces the separation.

Fig. 3. A typical SCBD apparatus set-up. The apparatus consists of two differentially pumped vacuum vessels separated by a skimmer for the molecular beam. In first chamber a PMCS cluster source is shown [62], equipped with a focusing nozzle. A cluster–buffer gas mixture expands through the nozzle forming a free jet and accelerating the nanoparticles to hyper-thermal kinetic energy. The skimmer selects only the central portion of the expanding jet and allows to keep the deposition chamber at high vacuum conditions. The use of a focusing nozzle allows to concentrate the particles within the angle of acceptance of the skimmer aperture.

Fig. 4. A thickness map showing the intensity distribution of particles inside a cluster beam generated by a PMCS. The characteristic shape is due to depletion from particles of the flow lines close to the cylindrical cathode surface.