A schematic of the experimental system is shown in the figure below (click for a full-featured animation). To suspend a micron sized charged particle in air within a small region an electrodynamic balance is used. Here, a Millikan type DC field acting against the effect of gravitation is superimposed with a non uniform alternating field which has a focusing effect. The charged particle is held in space in the null point of the non uniform field. The DC field is applied between two endcap electrodes and the alternating field to the two electrodes located above and below the midplane. The balance is placed in a three wall glass chamber, with a cooling liquid flowing through the inner walls and an isolation vacuum between the outer walls. A constant flow of an appropriate gas mixture is pumped continuously through the chamber, the total pressure can be varied between 50 and 1000 hPa.
Three collinear laser beams illuminate the particle from below (HeNe @ 633 nm, Ar+ @ 488 nm, and a Tunable-Diode-Laser @ 763-787 nm)Several methods are used to characterize the aerosol: the DC voltage applied to compensate the gravitational force is proportional to the mass of the particle, Mie phase functions are used to deduce the radius of the aerosol and to detect phase changes, Raman spectroscopy is employed to measure its composition, the radius and refractive index of a spherical particle can be measured with high precision using a tunable diode laser (TDL) or a "white" light point source LED and analyzing the Mie resonance spectra. Additionally, temporal light-scattering fluctuations and spatial asymmetry in the 2D scattering
patterns are used to deduce solid-to-liquid partitioning of the particles.
Presently, we are also investigating the possibility to use dynamic light scattering or photon correlation spectroscopy to characterize a non spherical particle.
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