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 = 50 μm) 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 d(b) as function of the control parameters are accounted for by a scaling theory, which predicts d(b)/w ∝ (μ(g)/μ(l))(1/12)(Q(g)/Q(l))(5/12), where μ(g) and μ(l) indicate, respectively, the gas and liquid viscosities and Q(g) and Q(l) are the gas and liquid flow rates. As a particularly important application of our results we produce monodisperse bubbles with the appropriate diameter for therapeutic applications (d(b) ≃ 5 μm) and a production rate exceeding 10(5) Hz.

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Electroactuation of liquid–liquid interfaces offers promising methods to actively modulate droplet for - mation 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 drop - let 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 maxi - mize 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

Bubbles created with ultrasound from artificial microscopic crevices can improve energy efficiency values for the creation of radicals; nevertheless it has been conducted so far only under special laboratory conditions. Limited reproducibility of results and poor energy efficiency are constraints for the sonochemistry and ultrasonics community to scale-up applied chemical processes. For the first time, using conventional ultrasonic bath technology, the numbering-up and scale-up of a microfluidic sonochemical reactor has been achieved. Sonochemical effects such as radical production and sonochemiluminescence were intensified by the modification of the inner walls of a novel Cavitation Intensification Bag. While 25 times bigger than the previous microreactor, a reduction of 22 % in standard deviation and an increase of 45.1 % in efficiency compared to bags without pits were obtained. Mechanical effects accompanying bubble collapse lead to two distinct types of erosion marks observed in the bags.Cavitation, the formation and collapse of bubbles in liquids, has been used as a green energy-focusing tool to produce chemical effects (notably production of free radicals), enhanced luminescence, mechanical activation of heterogeneous systems, physicochemical modifications of inert materials as well as water remediation, water splitting, and bioenergy applications. [1] These effects can all be harnessed in applied domains, from cleaning to water treatment and nanochemistry. However, a main barrier for sonochemical and ultrasonic reactors to be uti-lized for industrial purposes and other uses is the lack of reproducibility, along with a low energy efficiency. [2] Employing the same ultrasonic equipment, glassware, chemicals and experimentalist person, is no guarantee that the standard deviations of an expected result will be small. This lack of reproducibility is because creating bubbles with ultrasound closely resembles a stochastic process, depending on physical-chemical factors difficult to control at once. [3] The first is the nucleation sites from which bubbles are created. Once bubbles are nucleated, liquid-gas concentration, frequency and amplitude of ultrasound signal, etc. have a significant influence on the overall cavitation process. These parameters influence the "unitary" reactor that a bubble itself represents, and determine the generation of plasma conditions, sonoluminescence, shockwaves, jetting, and radical production, upon collapse. [4] Despite all these useful phenomena available at ambient pressure and room temperature, industrial applications have been hindered due to a meager~10 À6 -10 À5 (kg/kJ) energy efficiency of cavitation reactors. [5] The present work is motivated by the challenge in scalingup a microfluidic sonochemical reactor while increasing its results reproducibility. [6] The energy efficiency of that system was calculated as the product of radical formation rate and the energy required for the formation of OH . radicals divided by the electric power absorbed by the transducer. Wit...

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We report experiments in which a flow rate Q i of a fluid with a viscosity µ i discharges into an immiscible liquid of viscosity µ o that flows in parallel with the axis of the injector. When the outer capillary number verifies the conditionwhere U o and σ indicate, respectively, the outer velocity and the interfacial tension coefficient, and if the inner-to-outer velocity ratio is such thatwith R i the inner radius of the injector, a jet is formed with the same type of cone-jet geometry as predicted by the numerical results of Suryo & Basaran (Phys. Fluids, vol. 18, 2006, p. 082102). For extremely low values of the velocity ratio U i /U o , we find that the diameter of the jet emanating from the tip of the cone is so small that drops with sizes below 1 µm can be formed. We also show that, through this simple method, concentrated emulsions composed of micrometre-sized drops with a narrow size distribution can be generated. Moreover, thanks to the information extracted from numerical simulations of boundary-integral type and using the slender-body approximation due to Taylor (Proceedings of the 11th International Congress of Applied Mechanics, Munich, 1964, pp. 790-796), we deduce a third-order, ordinary differential equation that predicts, for arbitrary values of the three dimensionless numbers that control this physical situation, namely, Ca o , µ i /µ o and U i /U o , the shape of the jet and the sizes of the drops generated. Most interestingly, the influence of the geometry of the injector system on the jet shape and drop size enters explicitly into the third-order differential equation through two functions that can be easily calculated numerically. Therefore, our theory can be used as an efficient tool for the design of new emulsification devices.

The sloshing of liquids in microgravity is a relevant problem of applied mechanics with important implications for spacecraft design. A magnetic settling force may be used to avoid the highly non-linear dynamics that characterize these systems. However, this approach is still largely unexplored. This paper presents a quasi-analytical low-gravity sloshing model for magnetic liquids under the action of external inhomogeneous magnetic fields. The problems of free and forced oscillations are solved for axisymmetric geometries and loads by employing a linearized formulation. The model may be of particular interest for the development of magnetic sloshing damping devices in space, whose behavior can be easily predicted and quantified with standard mechanical analogies.

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