PALS
Phase Analysis Light Scattering (PALS)
Measurement of slow particle translations in ac-fields
Common dielectric particle spectroscopy methods analyze the frequency behavior of the field-induced motion microscopically to obtain information on the particles' internal structures. Phase analysis light scattering (PALS) was developed as an extension of the classical Laser-Doppler-Anemometry (LDA) for registration of extremely slow translational particle motion.
The figure schematically shows the optical setup of our PALS-device. The measuring volume is defined by the interference pattern in the crossing region of the two Laser beams. The Bragg cells in the beam path introduce a small (kilohertz range) difference of the optical frequency of the two beams. This small frequency shift causes a frequency offset in the scattered Doppler signal which is given by the frequency difference of the beams.
Since the phase of the Doppler signal is related to the particle's position, particle displacement is reflected by a change of the the phase of the scattered light. A translational particle motion can be induced in inhomogeneous fields (dielectrophoresis, DP) [51] or by traveling wave fields generated in arrays of repetitively arranged electrodes (traveling wave dielectrophoresis, TWDP). To improve statistics, amplitude weighting of the measured phase differences was used.
PALS allows the registration of very small particle displacements otherwise overwhelmed by random Brownian motion. The advantage is that frequency-spectra of particle translation induced by DP or TWDP can be registered at low field strengths. Our method overcomes the limitation of microscopic methods and allows computerization of the measuring procedure.
More information: Jan Gimsa