# Concentrated suspensions of Brownian beads in water: dynamic heterogeneities trough a simple experimental technique

## TL;DR

This paper compares a novel optical technique, Differential Variance Analysis (DVA), with traditional particle tracking to study dynamic heterogeneities in dense suspensions of Brownian beads, revealing both similarities and quantitative differences near the glass transition.

## Contribution

It demonstrates the effectiveness of DVA in characterizing dynamic heterogeneities and compares its results with single-particle tracking in glassy suspensions.

## Key findings

- DVA successfully monitors structural relaxation and heterogeneities.
- Qualitative agreement between DVA and particle tracking on heterogeneity signatures.
- Quantitative discrepancies in time and length scales near the glass transition.

## Abstract

Concentrated suspensions of Brownian hard-spheres in water are an epitome for understanding the glassy dynamics of both soft materials and supercooled molecular liquids. From an experimental point of view, such systems are especially suited to perform particle tracking easily, and, therefore, are a benchmark for novel optical techniques, applicable when primary particles cannot be resolved. Differential Variance Analysis (DVA) is one such novel technique that simplifies significantly the characterization of structural relaxation processes of soft glassy materials, since it is directly applicable to digital image sequences of the sample. DVA succeeds in monitoring not only the average dynamics, but also its spatio-temporal fluctuations, known as dynamic heterogeneities. In this work, we study the dynamics of dense suspensions of Brownian beads in water, imaged through digital video-microscopy, by using both DVA and single-particle tracking. We focus on two commonly used signatures of dynamic heterogeneities: the dynamic susceptibility, $\chi_4$, and the non-Gaussian parameter, $\alpha_2$. By direct comparison of these two quantities, we are able to highlight similarities and differences. We do confirm that $\chi_4$ and $\alpha_2$ provide qualitatively similar information, but we find quantitative discrepancies in the scalings of characteristic time and length scale on approaching the glass transition.