# Direct measurement of the ballistic motion of a freely floating colloid in Newtonian and viscoelastic fluids

## TL;DR

This study directly measures the ballistic-to-diffusive transition of a colloid in Newtonian and viscoelastic fluids, revealing agreement with models in Newtonian fluids and significant deviations in viscoelastic fluids, providing insights into microscopic fluid properties.

## Contribution

It provides the first direct measurements of the ballistic-to-diffusive transition in viscoelastic fluids and compares experimental results with theoretical models.

## Key findings

- Agreement with Clercx-Schram model in Newtonian fluids
- Discrepancies between theory and experiment in viscoelastic fluids
- Observation of increased effective mass and decreased plateau modulus in viscoelastic fluids

## Abstract

A thermal colloid suspended in a liquid will transition from a short time ballistic motion to a long time diffusive motion. However, the transition between ballistic and diffusive motion is highly dependent on the properties and structure of the particular liquid. We directly observe a free floating tracer particle's ballistic motion and its transition to the long time regime in both a Newtonian fluid and a viscoelastic Maxwell fluid. We examine the motion of the free particle in a Newtonian fluid and demonstrate a high degree of agreement with the accepted Clercx-Schram model for motion in a dense fluid. Measurements of the functional form of the ballistic-to-diffusive transition provide direct measurements of the temperature, viscosity, and tracer radius. We likewise measure the motion in a viscoelastic Maxwell fluid and find a significant disagreement between the theoretical asymptotic behavior and our measured values of the microscopic properties of the fluid. We observe a greatly increased effective mass for a freely moving particle and a decreased plateau modulus.