One-dimensional microfluidic crystals far from equilibrium acoustic phonons, instabilities and confinement

We investigated the collective motion of a one-dimensional array of water-in-oil droplets flowing in microfluidic channel in quasi-2D at low Reynolds number. Driven far from equilibrium by the symmetry-breaking flow field, the droplets exhibit acoustic normal modes (crystal 'phonons') with unusual dispersion relations. These phonons are due to long-range hydrodynamic dipolar interactions between the droplets. The phonon spectra change anomalously at the crossover between unconfined 2D flow and ID confined flow, as a result from an interplay between boundary-induced screening and crystal incompressibility. Microfluidic crystals offer a vista, in the linear flow regime, into soft-matter systems far from equilibrium.