Patent GB-1410999-A (GB)
Title: Electrostatic levitators for charged particles
IPC: G01N21/53
Filed: 1972-10-12
Abstract: 1410999 Photo-electric control of levitator for light scattering measurements SCIENCE SPECTRUM Inc 12 Oct 1972 [21 Oct 1971] 47211/72 Heading G1A [Also in Division H1] An electrostatic levitator Figs. 1a, 1b, lc comprises parallel electrodes 10, 12 with a third elongated electrode 14 extending through 10 and insulated therefrom; differing potentials being applied to the several electrodes to deflect a negatively charged particle and being reversed in sense to deflect a positively charged particle. The P.D. between 12 and 14 produces a field deflecting the particle upwards and inwards and that between 10, 14 deflects it radially inwards so that for zero potential on 12 the particle would be attracted towards electrode 14 and for zero potential on 14 it would be attracted downwards towards 12 in reinforcement of gravity. In combination, a radial field 20 centres particle 16 and a weaker combined vertical field of 18, 20 counterbalances gravity. The electrode 14 may extend through the lower electrode 12 and the plane of electrodes 10, 12 need not be perpendicular to gravity. The device is applicable to a sealed scattering cell detecting differential light scattering by microparticles, which comprises Fig. 2 a cover 100 with inlet connector 102 and flushing connector 104, sealedly fitting to a chamber 106 containing the particles whose base 10 is the upper electrode with electrode 14 extending through insulator 108 fitting opening 110, and energized over connector 112 on the outer wall. Electrode 10 and the chamber are earthed, and a bellows connector 116 is fitted in the chamber wall, while aperture 118 of the insulator 108 allows particles to pass into a sealed lower light scattering cell having a transparent member 120 masked at 122, into which laser light passes over entrance mask 124 and is received by light trap 126. A part 128 angled to the axis enables microscope observation. Differential light scattering by a microparticle located in the transparent cell is detected by viewing it at different angles through the transparent cell 120, whose sealed base 132 supports lower electrode 12, excited over connector 140; while an exhaust connector 138 communicates with the interior. Microparticles produced by a pneumatic nebulizer Fig. 3 (not shown), are introduced through the inlet connector 102, and air may alternatively be injected into the flushing connector 104; while low-pressure air is supplied from a  bellows to connector 116 and the cell is exhausted over a hose from connector 138. Electrode potantials and polarities, injection of flushing and bellows air and nebulizer operation are controlled from a panel Fig. 4 (not shown), incorporating an inspection microscope so that a particle may be positionally adjusted in the cell. The optical system Fig. 5 comprises a lens focusing the light emanent from a scattering particle through aperture 128 on to beam splitter 304 and mirror 310 for inspection by microscope 308, while other light is reflected by mirrors 306 and 312 to photomultipliers 314, 316 combining to detect the vertical position of a particle in the cell in response to the differential scattered light received by the photo-multipliers, whose outputs are equal when the particle is in the desired position. The outputs energize matched balanced logarithmic amplifiers whose outputs are differenced, amplified, and applied to a sample and hold circuit switchably controlled by a one shot multi-vibrator in response to the outputs of the respective amplifiers. The sample and hold circuit output is amplified to operate a pair of D.C.-D.C. converters energizing electrodes over a polarity switch to control the particle position. An alternative manual positional control is provided Fig. 6 (not shown). In operation, the nebulizer is charged with a suspension of microparticles, the laser and electrodes are energized with appropriate voltage and polarity, and the particle manual positional control is engaged. The cell is flushed with clean air until no particles are visible in the microscope, the nebulizer is operated to inject microparticles, which are moved through the laser beam by the bellows. Voltage and position control are adjusted to select a particle to be held in the microscope field, after which automatic control may be switched in to hold it positioned in the laser beam. Scattered light is intercepted by a periscopic detector (not shown), movable in an arc surrounding the particle, which operates a recorder plotting scattered light intensity relative to incident light. USA Specifications 3624835 and 3730842 are referred to.