Abstract：The principle of DDM was introduced and used to study the Brownian motion of sub-micro particles in water. A white light, a high speed camera and a microscope were used to obtain particle images experimentally. The analysis software was employed to process the original images. The experimental results show that 1) in the spatial domain, with increasing the delay time, the increase trend of differential signal reflects the particles rearrangement due to their Brownian motion. 2) In the frequency domain, with increasing the delay time, the increase trend of differential signal corresponding to different wave vector is in good agreement with that in the spatial domain. Furthermore, the characteristic time corresponding to different wave vector decreases with increasing the wave vector. 3) The deviation between calculated diffusion coefficient of particles in water and the theoretically expected values based on the Stokes-Einstein equation is 4.6%. The experimental results can reflect Brownian motion characteristics of sub-micro particles accurately.

Keywords: Brownian motion; differential dynamic microscopy; diffusion coefficient