Micro-confined droplets: From lubrication film to droplet velocity

Abstract eng:
We study the motion of droplets in a confined, micrometric geometry, by focusing on the lubrication film effect on droplet velocity. When capillary forces dominate, the lubrication film thickness evolves non-linearly with the capillary number due to the viscous dissipation between the meniscus and the wall. However, this film may become thin enough (tens of nanometres) that intermolecular forces come into play and affect classical scalings. Our experiments yield highly resolved topographies of the shape of the interface and allow us to bring new insights into droplet dynamics in microfluidics. We report the novel characterization of two dynamical regimes as the capillary number increases: (i) at low capillary numbers, the film thickness is constant and set by the disjoining pressure, while (ii) above a critical capillary number, the interface behaviour is well described by a viscous scenario. Considering this, we propose a refined model to predict the droplet velocity.

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