Nano-particle characterization by using Exposure Time Dependent Spectrum and scattering in the near field methods: how to get fast dynamics with low-speed CCD camera

Light scattering detection in the near field, a rapidly expanding family of scattering techniques, has recently proved to be an appropriate procedure for performing dynamic measurements. Here we report an algorithm, first suggested by Oh et al. (Phys. Rev. E 69, 21106 (2004)), based on the evaluation of the Exposure Time Dependent Spectrum (ETDS), which makes it possible to measure the fast dynamics of a colloidal suspension with the aid of a simple near field scattering apparatus and a CCD camera. The algorithm consists in acquiring static spectra in the near field at different exposure times, so that the measured decay times are limited only by the exposure time of the camera and not by its frame rate. The experimental set-up is based on a modified microscope, where the light scattered in the near field is collected by a commercial objective, but (unlike in standard microscopes) the light source is a He-Ne laser which increases the instrument sensitivity. The apparatus and the algorithm have been validated by considering model systems of standard spherical nano-particle.