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From Foams to Phonons : Dynamic reorganizations of droplet arrangements
Droplet based microfluidics is well known for high throughput screening and the fabrication of special materials. But droplet based microfluidics also offers a convenient environment to study complex dynamics of dissipative, non-equilibrium many bodies systems as the droplet density and droplet confinement can be adjusted freely and the droplets can be observed easily by optical inspection.
If the volume fraction of a monodisperse emulsion is very small, the droplets in a confining geometry assemble into well-defined arrangements, analogous to foam. Hence the position of every droplet within an ensemble of droplets is fully determined. However, when lowering the droplet packing fraction below a critical value a droplet arrangement can be mechanically unstable and separates into different arrangements. It will be discussed how the mechanical unstable arrangements lead to oscillations or packing density fluctuations in flowing trains of droplets. When lowering the packing fraction even further, so that the droplets do not touch each other, the droplets experience each other by hydrodynamic interactions and can develop a collective behavior which can be explained by a phonon-type behavior.