TIME SERIES ANALYSIS ON DILUTED SUSPENSIONS– PARTIAL RESULTS

D. Chicea

Faculty of Sciences,  University Lucian Blaga, Dr. Ion Ratiu Str. nr. 7-9, Sibiu, 550012, Romania

When coherent light is scattered on a biological suspension, a typical image named biospeckle is generated [1,2]. The time dependent speckle pattern obtained by stochastic interference of coherent light, scattered by microparticles in suspension was recorded using a photodetector and a fast data acquisition system. The measured electric voltage presents fluctuations that describe the microparticles dynamics. The sedimentation in gravitational field is the source of the slow fluctuations with large amplitude and small frequencies while the random Brownian motion gives the fast fluctuations with small amplitudes and bigger frequencies [3,4].
Starting from these results that have already been published [5,6], the time series recorded on a location in the far field were analyzed using the Fourier technique. Samples like milk, at different time intervals after being sampled and orange juice, in aqueous suspension at different concentrations and erythrocytes in isotonic solution were analyzed. The amplitude frequency spectrum for each sample has several peaks at certain frequencies in the range 500-3500 Hz. So far it was found that the frequency peak positions do not significantly depend of the scattering center concentration. It was also found that the peaks frequency decreases as the scattering centers size increases.
The Fourier analysis of the time dependent biospeckle pattern suggests that each scattering center size in a suspension has a characteristic frequency sequence. This feature enables the appearance of particles with different size identification in a suspension. These results can be used in laboratory medicine (identification of bacterial population growth) and food quality control (decay or quality change of certain foods).

References
1.	A. Serov, W. Steenbergen, and F. de Mul, Prediction of the photo-detector signal generated by Doppler-induced speckle fluctuations: theory and some validations, J. Opt. Soc. Am. A 18, (2001) 622-630.
2.	J. D. Briers, Laser Doppler, speckle and related techniques for blood perfusion mapping and imaging, Physiol. Meas. 22, (2001) R35–R66.
3.	C.V.L. Pop, C. Vamos, I. Turcu, Numerical simulation of speckled light scattered by red blood cells in suspensions, 4th European Biophysics Congress, July 5-9,Alicante, Spain, Eur. Biophys. J. 32, 3 (2003) 220.
4.	I. Turcu, C. V. L. Pop, S. Neamtu, L. Ciortea; Crossover from single to multiple light scatter in erythrocytes suspensions, Applied Optics (submitted) 2004;
5.	I. Turcu, C. V. L. Pop, S. Neamtu, Intensity Fluctuating Pattern of Light Scattered on Microparticles, Studia Universitatis Babeş-Bolyai, Physica, Special Issue, 2003.
6.	C.V.L. Pop, I. Turcu, C. Vamos, Simulation of fluctuating interference pattern for light scattered on erythrocytes in suspension, Studia Universitatis Babeş-Bolyai, Physica, Special Issue, 2003.