Experimental and Numerical Studies on Microfluidic Systems

Abstract

The work presented in this thesis can be summarized as a compilation of five different and comprehensive studies in the field of microuidic ows, related to the formation of jets, drops and bubbles; where the surface tension plays a major role. The topics studied are classified in chapters where the problem formulation, procedures, and results are individually presented. Chapter 2 is devoted to understand the evolution of Newtonian capillary jets and to study the instability transition of viscoelastic jets under axisymmetric perturbations. A mathematical model has been used to determine the parameter conditions for which the convective to absolute instability transition takes place, playing special attention to the role played by unrelaxed elastic axial stress. Chapter 3 presents results of a numerical study of rivulets in microchannels in order to characterize stable and unstable regimes. The theoretical frame work and stability analysis are presented in detail. It was found that a basic ow can become unstable when that quantity exceeds a certain critical value, while the rest of governing parameters remain constant. Chapter 4 discuses a ubiquitous process in science and technology - the dissolution of microbubbles. As in the previous chapters, detailed theoretical and numerical approaches are developed from scratch, culminating in a set of carefully performed experiments. Numerical and experimental results agree well and complement each other. We move then onto Chapter 5 which studies the electrical disruption of pendant liquid drops. The focus of the study here is the behaviour of suddenly electrified pendant droplets in dielectric liquid. Supported by numerical and experimental results, we argue that the viscosity of the surrounding uid is responsible for the development of more complex jetting processes such as what is called splashing in which the tip of the cone explodes onto a mushroom-like structure, and splitting regimes. Moreover, when the cone evolves into one of these modes they do it in a way that is dependent on the large scale properties such as the initial droplet size and on the applied voltage - contrary to the well-established (universal) mechanisms encountered in the tip streaming mode. Finally, Chapter 6 presents a series of oneto- one numerical and experimental runs with excellent agreement of a novel way of producing drops that are signi_cantly smaller than the nozzle from which they emerge. A very detailed discussion of the experimental rig is presented, including the important parameters to be taken in account, such as the meniscus formed at the nozzle and the deformation of the nozzle plate during the driving pressure pulses. Finally, a predictive scaling law of the produced droplet size was obtained.