On the role of initial velocities in pair dispersion in a microfluidic chaotic flow

TL;DR

This study investigates how initial velocities influence pair dispersion in microfluidic elastic turbulence, revealing that ballistic separation dominates over long times and scales, challenging previous assumptions of flow smoothness.

Contribution

It provides the first three-dimensional experimental evidence of ballistic pair dispersion in elastic turbulence at small scales, highlighting the breakdown of flow smoothness assumptions.

Findings

Ballistic separation dominates long-term pair dispersion.

Flow smoothness breaks at scales smaller than one-tenth of the system size.

Ballistic dispersion is a universal phenomenon in small-scale chaotic flows.

Abstract

Chaotic flows drive mixing and efficient transport in fluids, as well as the associated beautiful complex patterns familiar to us from our every day life experience. Generating such flows at small scales where viscosity takes over is highly challenging from both the theoretical and engineering perspectives. This can be overcome by introducing a minuscule amount of long flexible polymers, resulting in a chaotic flow dubbed \textit{elastic turbulence}. At the basis of the theoretical frameworks for its study lie the assumptions of a spatially smooth and random-in-time velocity field. Previous measurements of elastic turbulence have been limited to two-dimensions. Using a novel three-dimensional particle tracking method, we conduct a microfluidic experiment, allowing us to explore elastic turbulence from the perspective of particles moving with the flow. Our findings show that the smoothness assumption breaks already at scales smaller than a tenth of the system size. Moreover, we provide conclusive experimental evidence that \textit{ballistic} separation prevails in the dynamics of pairs of tracers over long times and distances, exhibiting a memory of the initial separation velocities. The ballistic dispersion is universal, yet it has been overlooked so far in the context of small scales chaotic flows.