Mode coupling of phonons in a dense one-dimensional microfluidic crystal

Long-living coupled transverse and longitudinal phonon modes are explored in dense, regular arrangements of flattened microfluidic droplets. The collective oscillations are driven by hydrodynamic interactions between the confined droplets and can be excited in a controlled way. Experimental results are quantitatively compared to simulation results obtained by multi-particle collision dynamics. The observed transverse modes are acoustic phonons and obey the predictions of a linearized far-field theory. The longitudinal modes arise from a nonlinear mode coupling due to the lateral variation of the confined flow field. The proposed mechanism for the nonlinear excitation is expected to be relevant for hydrodynamic motion in other crowded non-equilibrium systems under confinement.