The possibility of laser Doppler velocimetry (LDV) in microchannels, where particles are suspended in a liquid, and where oscillating or evanescent guided coherent light wave is present, is examined theoretically. The conditions for the observation of the transverse and longitudinal collective phoretic particle movement using the time correlation function (TCF) of the fluctuations of the light scattered by particles suspended in the channel or its frequency spectrum (FS) are formulated. It is shown that for the observation of the longitudinal phoresis, the heterodyne regime where the light scattered by particles is mixed with a reference wave of the observation should be employed. The study carried out shows that heterodyne regime establishment requires a multimode waveguide, where mode coupling can inherently occur. For the observation of the transverse phoretic movement, the homodyne regime, where only the light scattered by particles reaches the detector, may also be employed. The TCF or FS of the light scattered by suspended particles may be computed using the light intensity signal in different guided modes containing fluctuations if the coupling coefficients are influenced by the electric field fluctuations due to presence of the scattering particles. After the computation of the TCF is carried out, this TCF or its FS may be used to obtain the information on the particle or macromolecule phoretic movement and diffusion in the channel. The information may be used for the electrophoretic analysis of the multicomponent liquid systems in the miniature integrated optical devices with the continuous introduction of the analyzed liquid mixture.
Pandraud, G., van den Berg, A., & Semenov, S. N. (2000). Laser Doppler velocimetry in Microchannels using integrated optical waveguides. Optics communications, 174(174), 355-381. https://doi.org/10.1016/S0030-4018(99)00699-9