Differential Dynamic Microscopy to characterize Brownian motion and bacteria motility

TL;DR

This paper introduces Differential Dynamic Microscopy (DDM) as an educational tool for undergraduate students to quantify microscopic particle dynamics, including Brownian motion and bacteria motility, using standard microscopy and digital analysis.

Contribution

It presents a comprehensive tutorial on DDM, demonstrating its application to colloids and bacteria, with code implementations in Matlab and Python for educational purposes.

Findings

DDM accurately measures diffusion coefficients in colloids.

DDM characterizes bacteria motility parameters.

The method is accessible for undergraduate teaching.

Abstract

We have developed a lab work module where we teach undergraduate students how to quantify the dynamics of a suspension of microscopic particles, measuring and analyzing the motion of those particles at the individual level or as a group. Differential Dynamic Microscopy (DDM) is a relatively recent technique that precisely does that and constitutes an alternative method to more classical techniques such as dynamics light scattering (DLS) or video particle tracking (VPT). DDM consists in imaging a particle dispersion with a standard light microscope and a camera. The image analysis requires the students to code and relies on digital Fourier transform to obtain the intermediate scattering function, an autocorrelation function that characterizes the dynamics of the dispersion. We first illustrate DDM on the textbook case of colloids where we measure the diffusion coefficient. Then we show that DDM is a pertinent tool to characterize biologic systems such as motile bacteria i.e.bacteria that can self propel, where we not only determine the diffusion coefficient but also the velocity and the fraction of motile bacteria. Finally, so that our paper can be used as a tutorial to the DDM technique, we have joined to this article movies of the colloidal and bacterial suspensions and the DDM algorithm in both Matlab and Python to analyze the movies.