Observation of Two-Dimensional Brownian Motion by Microscope Image Sequence Processing

Abstract

We propose a method to measure Brownian motion based on image sequence processing. Random motion of sub-micron sphere particles is visualized under an inverted microscope with laser light illumination. We analyze a long image sequence of the motion by a spatial-filtering method, which corresponds to dynamic light scattering. We confirm that bigger particle (diameter=1.09 μm) show ideal Brownian motion with an inverse power-law spectrum P(f)∝ f^-2. In tiny particles (diameter=0.46 and 0.20 μm), however, we observe a deviation from f^-2 behavior. When the motion of particles is limited within a two-dimensional plane by use of heavy water, ordinary behavior of f^-2 spectrum is recovered. We confirm high reliability and big advantages of the proposed method compared to dynamic light scattering.