Collective waves in dense and confined microfluidic droplet arrays

TL;DR

This study explores how collective wave patterns in dense, confined microfluidic droplet arrays can be excited and controlled, revealing complex hydrodynamic interactions and potential applications in tunable metamaterials.

Contribution

It demonstrates the excitation of specific collective modes via defect patterns and uncovers the hydrodynamic coupling mechanisms in dense droplet arrays.

Findings

Distinct collective wave modes can be excited by defect patterns.

Hydrodynamic interactions lead to mode coupling and dispersion relations.

Potential for dynamically tunable optofluidic metamaterials.

Abstract

Excitation mechanisms for collective waves in confined dense one-dimensional microfluidic droplet arrays are investigated by experiments and computer simulations. We demonstrate that distinct modes can be excited by creating specific `defect' patterns in flowing droplet trains. Excited longitudinal modes exhibit a short-lived cascade of pairs of laterally displacing droplets. Transversely excited modes obey the dispersion relation of microfluidic phonons and induce a coupling between longitudinal and transverse modes, whose origin is the hydrodynamic interaction of the droplets with the confining walls. Moreover, we investigate the long-time behaviour of the oscillations and discuss possible mechanisms for the onset of instabilities. Our findings demonstrate that the collective dynamics of microfluidic droplet ensembles can be studied particularly well in dense and confined systems. Experimentally, the ability to control microfluidic droplets may allow to modulate the refractive index of optofluidic crystals which is a promising approach for the production of dynamically programmable metamaterials.