Local characterization of hindered Brownian motion by using digital video microscopy and 3D particle tracking

TL;DR

This paper introduces methods for analyzing hindered Brownian motion in complex 3D geometries using digital video microscopy and particle tracking, enabling detailed local diffusion measurements in heterogeneous systems.

Contribution

The authors develop a local measurement approach for diffusion, tracking accuracy, and drift velocities in 3D, improving analysis of heterogeneous environments over traditional global methods.

Findings

Measured local diffusion coefficients in microfluidic channels.

Achieved accurate 3D particle tracking in complex geometries.

Validated methods for particles under external forces.

Abstract

In this article we present methods for measuring hindered Brownian motion in the confinement of complex 3D geometries using digital video microscopy. Here we discuss essential features of automated 3D particle tracking as well as diffusion data analysis. By introducing local mean squared displacement-vs-time curves, we are able to simultaneously measure the spatial dependence of diffusion coefficients, tracking accuracies and drift velocities. Such local measurements allow a more detailed and appropriate description of strongly heterogeneous systems as opposed to global measurements. Finite size effects of the tracking region on measuring mean squared displacements are also discussed. The use of these methods was crucial for the measurement of the diffusive behavior of spherical polystyrene particles (505 nm diameter) in a microfluidic chip. The particles explored an array of parallel channels with different cross sections as well as the bulk reservoirs. For this experiment we present the measurement of local tracking accuracies in all three axial directions as well as the diffusivity parallel to the channel axis while we observed no significant flow but purely Brownian motion. Finally, the presented algorithm is suitable also for tracking of fluorescently labeled particles and particles driven by an external force, e.g. electrokinetic or dielectrophoretic forces.