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"En las siguientes secciones del presente capítulo se van a desarrollar las distintas partes que componen el código del programa objeto de este proyecto. El programa ha sido implementado en una interfaz gráfica de usuario en Matlab para el procesado de vídeos de experimentos de microfluídica, para el análisis de burbujas y chorros en conductos o canales." (Fragmento extraido de la Introducción del trabajo)

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

A novel swirl flow-focusing microfluidic axisymmetric device for the generation of monodisperse microbubbles at high production rates to be used as in-line contrast agents for medical applications is presented. The swirl effect is induced upstream of the discharge orifice by a circular array of microblades which form a given angle with the radial direction. The induced vortical component on the focusing liquid stabilizes the gas meniscus by the vorticity amplification due to vortex stretching as the liquid is forced through the discharge orifice. The stabilized meniscus tapers into a steady gas ligament that breaks into monodisperse microbubbles. A reduction up to 57% in the microbubble diameter is accomplished when compared to conventional axisymmetric flow-focusing microdevices. An exhaustive experimental study is performed for various blade angles and numerous gas to liquid flow rate ratios, validating previous VoF numerical simulations. The microbubbles issued from the stabilized menisci verify prior scaling law of flow-focusing.

We report the production of droplet groups with a controlled number of drops in a microfluidic electro-flow focusing device under the action of an AC electric field. This regime appears for moderate voltages (500-700 V peak-to-peak) and signal frequencies between 25 and 100 Hz, much smaller than the droplet production rate ( ≈500 Hz). For this experimental conditions the production frequency of a droplet package is twice the signal frequency. Since the continuous phase flow in the microchannel is a Hagen-Poiseuille flow, the smaller droplets of a group move faster than the bigger ones leading to droplet clustering downstream.

We use a microfluidic flow-focusing device with integrated electrodes for controlling the production of water-in-oil drops. In a previous work, we reported that very long jets can be formed upon application of AC fields. We now study in detail the appearance of the long jets as a function of the electrical parameters, i.e., water conductivity, signal frequency, and voltage amplitude. For intermediate frequencies, we find a threshold voltage above which the jet length rapidly increases. Interestingly, this abrupt transition vanishes for high frequencies of the signal and the jet length grows smoothly with voltage. For frequencies below a threshold value, we previously reported a transition from a well-behaved uniform jet to highly unstable liquid structures in which axisymmetry is lost rather abruptly. These liquid filaments eventually break into droplets of different sizes. In this work, we characterize this transition with a diagram as a function of voltage and liquid conductivity. The electrical response of the long jets was studied via a distributed element circuit model. The model allows us to estimate the electric potential at the tip of the jet revealing that, for any combination of the electrical parameters, the breakup of the jet occurs at a critical value of this potential. We show that this voltage is around 550 V for our device geometry and choice of flow rates.

Electroactuation of liquid-liquid interfaces offers promising methods to actively modulate droplet formation in droplet-based microfluidic systems. Here, flow-focusing junctions are coupled to electrodes to control droplet production in the well-known jetting regime. In this regime, a convective instability develops leading to droplet formation at the end of a thin and uniform, long liquid finger. We show that in AC electric fields the jet length is a function of both the magnitude of the applied voltage and the electrical parameters such as the frequency of the AC field and the conductivity of the dispersed phase.We explain that dependency using a simple transmission line model along the liquid jet. An optimum frequency to maximize the liquid ligament length is experimentally observed. Such length simply cannot be obtained by other means under the same operating conditions, in the absence of the AC signal. At low frequency, we reach a transition from a well-behaved, uniform jet brought about near the optimum frequency to highly unstable liquid structures in which axisymmetry is lost rather abruptly

La presente tesis doctoral trata de profundizar en los mecanismos de generación y rotura tanto de gotas como de burbujas empleando distintas configuraciones.

A new regime of operation of PDMS-based flow-focusing microfluidic devices is presented. We show that monodisperse microbubbles with diameters below one-tenth of the channel width (here w = 50µm) can be produced in low viscosity liquids thanks to a strong pressure gradient in the entrance region of the channel. In this new regime bubbles are generated at the tip of a long and stable gas ligament whose diameter, which can be varied by tuning appropriately the gas and liquid flow rates, is substantially smaller than the channel width. Through this procedure the volume of the bubbles formed at the tip of the gas ligament can be varied by more than two orders of magnitude. The experimental results for the bubble diameter db as function of the control parameters are accounted for by a scaling theory, which predicts db/w ∝ (µg/µℓ)1/12 (Qg/Qℓ )5/12 , where µg and µℓ indicate, respectively, the gas and liquid viscosities and Qg and Qℓ are the gas and liquid flow rates. As a particularly important application of our results we produce monodisperse bubbles with the appropriate diameter for therapeutical applications (db ≃5µm) and a production rate exceeding 105 Hz.

We perform extensive experiments with coflowing liquids in microfluidic devices and provide a closed expression for the drop size as a function of measurable parameters in the jetting regime that accounts for the experimental observations; this expression works irrespective of how the jets are produced, providing a powerful design tool for this type of experiments

Procedimiento de preparación de partículas de tamaño micro y nanométrico con productos lábiles y partículas obtenidas. La presente invención esta relacionada con la obtención de partículas poliméricas de tamaño micro y nanométrico de una forma controlable y reproducible. Las partículas tienen forma esférica y una distribución de tamaño muy estrecha y homogénea. Mas particularmente, la presente invención describe el use y utilización de un método suave de formación de partículas y su aplicación a la encapsulación de compuestos frágiles de interés biológico, incluyendo desde péptidos y proteínas hasta células y microorganismos.