  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.
