Forcing microbubbles in microfluidics

The present thesis is a compilation of three studies in the field of microfluidic, more concretely, the generation of microbubbles and the effect that different applied forces have on them. A microbubble generation state of the art in terms of applications, employed fluids, working regimes and microfluidic devices is introduced in the first place. Several microfluidic devices: cross junction, TJunction, planar and axisymmetric flow focusing are compared with regard to their operational woking regime -bubbling, jetting or squeezing- and achievable microbubble size, as well as their fundamental advantages and limitations. In the second chapter, a novel swirl flow-focusing microfluidic axisymmetric device for the generation of monodisperse microbubbles at high production rates is presented. By forcing a swirl effect on the liquid stream, a more stable production, as well as a microbubble size reduction -up to 57% compared to the axisymmetric flow focusing-, is achieved due to the enhanced gas meniscus stability. The swirl is shown to expand the bounds of the jetting mode inhibiting the bubbling mode. An experimental study is performed for various blade angles -0º, 40º, 60º and 80º- and numerous gas to liquid flow rate ratios, validating previous numerical simulations and previous flow-focusing scaling law proposed by Gañán- Calvo [Gañán-Calvo, Physical Review E, 2004, 69(2), 027301]. Chips with 60º blades exhibit the best combination of swirl effect and robustness against perturbations. Chapter three is devoted to the active control of microbubble size on planar flow-focusing devices by means of an acoustic streaming or mechanical excitation. Few numerical studies have been reported so far, despite the invaluable information that computational analysis can through on this topic. In this chapter, the microbubble generation is numerically analyzed for an ample range of acoustic accelerations and frequencies and for several contact angles. A bubble volume change of 20% wh