Dynamic Light Scattering Signal Conditioning for Data Processing

Abstract

When performing data acquisition for a Dynamic Light Scattering experiment, one of the most important aspect is the filtering and conditioning of the electrical signal. The signal is amplified first and then fed as input for the analog digital convertor. As a result a digital time series is obtained. The frequency spectrum is computed by the logical unit offering the basis for further Dynamic Light Scattering analysis methods. This paper presents a simple setup that can accomplish the signal conditioning and conversion to a digital time series.Key words: Dynamic Light Scattering, Signal Conditioning, Time Series. IntroductionNanostructured materials have several applications in biology and medicine [1], mainly for investigating and delivering useful substances to the living cells. The physical properties of the relevant nanofluids and nanoparticles are in close connection to the particle size and size distribution. Therefore, the techniques used for estimating these physical properties are of major importance.One method for determining the average particle size for nanoparticles found in a suspension is the Dynamic Light Scattering or Photon Correlation Spectroscopy. The theoretical foundation of this method is described in various papers, [2], [3], [4] being some of them. This method is an optical procedure which uses coherent light to monitor in a fast manner the particles found in a suspension. The particles act as scattering centers and become secondary light sources when the incident coherent light interacts with them. This is a process of elastic scattering. As the incident light is coherent, the secondary waves emitted by the scattering centers are also coherent and due to interference they produce a scattering field. When placing a screen or a detector at a certain angle and distance from the sample, the interference field will have minima and maxima, randomly distributed, with the visual appearance of "boiling speckles". The intensity of light in each point of this field carries information on the phase of all the scattering centers from the active volume of the sample. The intensity of the light scattered by each center is anisotropic and depends mainly on the size and on the shape of the respective center. This information can be found in the phase function, which can be represented by various computational models [5], [6], [7].The motion of the particles is complex, being consisted mainly on sedimentation and Brownian motion [2]. It has been shown [8] that the sedimentation velocity is few orders of magnitude smaller than the velocity of the Brownian motion, when the particles size is in the nanometer scale. As a consequence, the phase function is affected mainly be Brownian motion, and since this motion is in relation to the size of the particles via the Einstein -Stokes relation, information about the size of the particles can be extracted by studying this phase. For this, the method is recording a time series using a light detector. The light detector generates an electrical signal, conv...