Short communicationSimultaneous measurement of size and density of spherical particles using two-dimensional particle tracking analysis method

- The performance of two-dimensional particle tracking analysis (2D-PTA) was investigated.
- Particle movement was observed by following the motion of bright spots scattered from laser-irradiated particles.
- Two different particle movements, diffusion and sedimentation, were observed at the same time.
- The analysis of the movements enabled determining the size and density of spherical particles.

The motion of spherical particles in liquid phases was investigated using a two-dimensional particle tracking analysis (2D-PTA) method. The analysis of four different materials with this method revealed two different types of particle movements occurring simultaneously: Brownian diffusion and sedimentation. When these two different displacements occurred simultaneously, in the case of particles with a larger size or higher density, the sedimentation displacements were significantly faster than those associated with Brownian diffusion. The occurrence of a faster sedimentation compared to the Brownian diffusion severely affects the accuracy of size determination using PTA methods, as these approaches are based on the Stokes-Einstein relationship, in which the size of a particle is inversely proportional to its Brownian diffusion coefficient. In addition, we successfully demonstrated, for the first time, the potential of the novel PTA method for simultaneously determining the particle size and identifying the constituent material of a limited range of silica and gold particle sizes. The present method may play an important role in the development of new industrial and biological applications of materials.

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