Interface and bulk exchange: Single drops experiments and CFD simulations

The coaxial double capillary for controlling pendant drops is an interesting experimental technique for investigating adsorption kinetics and transport properties of soluble components in the bulk which can exchange with the interface. The key process of this technique is the exchange of the entire bulk of a droplet without any direct perturbations of the interface due to forced convection. Therefore efficient exchange conditions should be determined in order to provide results during the exchange protocol that can be used for a reliable data interpretation of the adsorption process. In this paper we report on the flow into a drop formed with a double capillary set-up, as visualized by using a dye and simulated by computational fluid dynamics (CFD) of the flow field and mass transfer during the drop bulk exchange. Regarding the experimental protocols, for better monitoring and control of the droplet size a pulse-like flow with different injection volumes (dV) and pulse intervals (dt) combined with drop profile analysis tensiometry (PAT1) was used. The visualized experimental concentration fields are confirmed by the concentration contours obtained from CFD simulations. Both approaches give access to estimate the moment when the dye arrives at the drop surface. The drop surface exchange is also monitored via the measured surface tension. The computational results for average drop interface concentration (DIC) and drop outlet concentration (DOC) are discussed for different conditions.