Anomalous Microfluidic Phonons Induced by the Interplay of Hydrodynamic Screening and Incompressibility

TL;DR

This paper studies how confinement affects phonon spectra in a microfluidic droplet crystal, revealing anomalous behaviors due to hydrodynamic interactions and flow conditions.

Contribution

It provides a theoretical and experimental analysis of phonon spectrum anomalies caused by hydrodynamic screening and incompressibility in confined microfluidic systems.

Findings

Phonon spectra change anomalously under confinement.

Sound velocity increases near the 1D limit.

Hydrodynamic interactions are screened and strengthened by boundaries.

Abstract

We investigate the acoustic normal modes ("phonons") of a 1D microfluidic droplet crystal at the crossover between 2D flow and confined 1D plug flow. The unusual phonon spectra of the crystal, which arise from long-range hydrodynamic interactions, change anomalously under confinement. The boundaries induce weakening and screening of the interactions, but when approaching the 1D limit we measure a marked increase in the crystal sound velocity, a sign of interaction strengthening. This non-monotonous behavior of the phonon spectra is explained theoretically by the interplay of screening and plug flow.