Dynamics of bubbles in microchannels : theoretical, numerical and experimental analysis

This thesis aims at contributing to the characterization of the dynamics of bubbles in microfluidics through modeling and experiments. Two flow regimes encountered in microfluidics are studied, namely, the bubbly flow regime and the Taylor flow regime (or slug flow). In particular, the first part of this thesis focuses on the dynamics of a bubbly flow inside a horizontal, cylindrical microchannel in the presence of surfactants using numerical simulations. A numerical method allowing to simulate the transport of surfactants along a moving and deforming interface and the Marangoni stresses created by an inhomogeneous distribution of these surfactants on this interface is implemented in the Level set module of the research code. The simulations performed with this code regarding the dynamics of a bubbly flow give insights into the complexity of the coupling of the different phenomena controlling the dynamics of the studied system. Fo example it shows that the confinement imposed by the microchannel walls results in a significantly different distribution of surfactants on the bubble surface, when compared to a bubble rising in a liquid of infinite extent. Indeed, surfactants accumulate on specific locations on the bubble surface, and create local Marangoni stresses, that drastically influence the dynamics of the bubble. In some cases, the presence of surfactants can even cause the bubble to burst, a mechanism that is rationalized through a normal stress balance at the back of the bubble. The numerical method implemented in this thesis is also used for a practical problem, regarding the artisanal production of Mezcal, an alcoholic beverage from Mexico. The second part of the thesis deals with the dynamics of a Taylor flow regime, through experiments and analytical modeling. An experimental technique that allows to measure the thickness of the lubrication film forming between a pancake-like bubble and the microchannel wall is developed. The method requires only a single instantaneous bright-field image of a pancake-like bubble translating inside a microchannel. In addition to measuring the thickness of the lubrication film, the method also allows to measure the depth of a microchannel. Using the proposed method together with the measurment of the bubble velocity allows to infer the surface tension of the interface between the liquid and the gas. In the last chapter of this thesis, the effect of buoyancy on the dynamics of a Taylor flow is quantified. Though often neglected in microfluidics, it is shown that buoyancy effects can have a significant impact on the thickness of the lubrication film and consequently on the dynamics of the Taylor flow. These effects are quantified using experiments and analytical modeling. This work was performed at Princeton University with Professor Howard A. Stone during a seven month stay