| Thu March 8th 2018 14:00 – 15:00 ZH286 | Seminar | Dynamics of a 2D droplet in a Hele-Shaw cell Benjamin Reichert |
Details:Despite the fact that droplet microfluidics is a growing field of research, the dynamics of these objects remain misunderstood. Indeed, a question as fundamental as predicting a droplet’s velocity while it is pushed by an external fluid at a given velocity is still not answered. Understanding the dynamics of a droplet requires to characterize the viscous dissipation mechanisms (friction) within the droplet and in the lubrication film. This dissipation is related to the shape and to the physicochemical properties of the interface separating the inner phase of the droplet from the outer phase. This work presents a characterization of the dynamics of 2D droplets in a Hele-Shaw cell, by taking advantage of the measurement of both the lubrication film profile by interference microscopy and the droplet velocity. Firstly, we study experimentally the influence of the droplet viscosity and surfactant concentration on the shape of the interface. The comparison between the topographies measured experimentally with models reveals that the use of a purely hydrodynamical approach, in order to derive the theoretical topography, only allows to recover the experimental topography if the system is surfactant free or if the droplet viscosity is high enough to overcome the Marangoni effect at the interface. In the other cases, the shape of the interface depends on the Marangoni stress exerted either locally or globally at the interface of the droplet. In a second part, the derivation of a theoretical model for the droplet velocity, based on the modeling of the lubrication film topographies measured experimentally, allows to recover quantitatively, and without any fitting parameter, the experimental data on droplet velocities. As a whole, we show that predicting droplet velocity is only possible in some textbook cases (droplet without surfactant, or highly viscous droplet), otherwise the full topography of the lubrication film has to be measured. | ||
