Particle Size Inversion Of Dynamic Light Scattering Measurement At Ultra-low Concentration Particles

Dynamic light scattering(DLS)is an effective method for measuring submicron and nanoparticles.It can obtain the light intensity autocorrelation function(ACF)by measuring the scattered light signal which fluctuates with time,and then inverts the intensity ACF to obtain the particle size and particle size distribution(PSD).However,DLS measurement is limited by concentration,the lower limit is determined by the average particle number in the scattering volume.At ultra-low concentrations,particle number fluctuations in the scattering volume cause the intensity ACF to add a slowly decayed non-Gaussian term(number fluctuation decay)over long delay periods,which contains particle size information.But,the normal DLS data processing method does not consider the number fluctuation decay,which leads to the serious distortion of the inversion results,and the strong false peaks appear in the PSD.Therefore,how to deal with non-Gaussian terms caused by number fluctuations is very important for particle measurement of DLS at ultra-low concentration.In this thesis,based on the analysis of the characteristics of intensity ACF,the internal mechanism of false peaks was explored,the problem of PSD inversion of ultra-low concentration nanoparticles was further studied from the aspects of removing the number fluctuation contribution and making full use of the number fluctuation contribution.The main research contents include:1.The decay characteristics of the intensity ACF in ultra-low concentration particle system are analyzed.At ultra-low concentration,the number of particles in the scattering volume is small,the measured intensity fluctuations arise not only from the Brownian motion of the particles,but also from the particle number changes in the scattering volume.This leads to the intensity ACF decays on two widely different time scales.The smaller time scale is related to the change in the phase of the field scattered by a moving Brownian particle,while the larger scale is determined by the fluctuations of the number of Brownian particles in the effective scattering volume.2.The mechanism of false peaks in particle size inversion at ultra-low concentration is studied.The intensity ACF at ultra-low concentration is composed of Brownian motion decay and number fluctuation decay,which no longer satisfies Siegert relation with the electric field ACF.However,the classical DLS does not consider the number fluctuation decay,and the electric field ACF was obtained by Siegert relation as usual,which leads to the error of the calculated electric field ACF and the mismatch with the kernel matrix.In essence,the number fluctuation decay is mistaken for Brownian motion decay caused by large particles.Therefore,resulting in false peaks in the inversed PSD.3.Particle size inversion at ultra-concentration based on non-Gaussian term separation is studied.To improve the measurement accuracy of DLS at ultra-low concentration,according to the different decay characteristics caused by particle Brownian motion and particle number fluctuations in the intensity ACF,it is proposed to identify and separate the non-Gaussian terms by differentiating the intensity ACF,and then invert the PSD using the ACF containing only Brownian motion decay.Simulation and experimental results show that,the non-Gaussian term separation method effectively eliminates the strong spurious peaks in particle size distribution,corrects the peak shift,and realizes the accurate inversion of particle size distribution under ultra-low concentration.4.Particle size inversion at ultra-concentration based on non-Gaussian autocorrelation function correction.Aiming at the problem that the measured intensity ACF is not consistent with the theoretical correlation model because the measured time is much smaller than the delay time required to characterize the particle number fluctuation.Based on the analysis of the main parameters affecting the amplitude and relaxation time of non-Gaussian terms,the measured data were compared with the theoretical model to determine the correction parameters of the model,and the virtual focusing beam waist radius parameter was proposed to modify the non-Gaussian terms,so as to achieve the fitting of the measured non-Gaussian intensity ACF.Thus,the kernel function reconstruction method can be used to extract the particle size information contained in the non-Gaussian term.the proposed method gives significantly improved PSD recovery accuracy with experimental data.Furthermore,the PSDs obtained have no obvious differences to those obtained from measurements at normal particle concentrations.Particle size measurement of ultra-low concentration particle system is always a difficulty in DLS technique.Particle number fluctuation is the key to limit the application of dynamic light scattering in the field of low concentration and has an important effect on the inversion results.In this thesis,through the analysis and study of the non-Gaussian term of the intensity ACF,new data processing methods suitable for low concentration dynamic light scattering are proposed to change the current situation that DLS can not accurately measure the particle size distribution at ultra-low concentration.It is helpful to extend the lower limit of concentration measurement and improve the measurement range of DLS technology.