Analytical considerations and potential-flow numerical simulations of the pinch-off of bubbles at high Reynolds numbers reveal that the bubble minimum radius, rn, decreases as tau proportional to r2n sqrt[1lnr2n], where tau is the time to break up, when the local shape of the bubble near the singularity is symmetric. However, if the gas convective terms in the momentum equation become of the order of those of the liquid, the bubble shape is no longer symmetric and the evolution of the neck changes to a rn proportional to tau1/3 power law. These findings are verified experimentally.

We provide a comprehensive and systematic description of the diverse microbubble generation methods recently developed to satisfy emerging technological, pharmaceutical, and medical demands. We first introduce a theoretical framework unifying the physics of bubble formation in the wide variety of existing types of generators. These devices are then classified according to the way the bubbling process is controlled: outer liquid flows (e.g., coflows, cross flows, and flow-focusing flows), acoustic forcing, and electric fields. We also address modern techniques developed to produce bubbles coated with surfactants and liquid shells. The stringent requirements to precisely control the bubbling frequency, the bubble size, and the properties of the coating make microfluidics the natural choice to implement such techniques.

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The global stability of laminar axisymmetric low-density jets is investigated in the low Mach number approximation. The linear modal dynamics is found to be characterised by two features: a stable arc branch of eigenmodes and an isolated eigenmode. Both features are studied in detail, revealing that, whereas the former is highly sensitive to numerical domain size and its existence can be linked to spurious feedback from the outflow boundary, the latter is the physical eigenmode that is responsible for the appearance of self-sustained oscillations in low-density jets observed in experiments at low Mach numbers. In contrast to previous local spatio-temporal stability analyses, the present global analysis permits, for the first time, the determination of the critical conditions for the onset of global instability, as well the frequency of the associated oscillations, without additional hypotheses, yielding predictions in fair agreement with previous experimental observations. It is shown that under the conditions of those experiments, viscosity variation with composition, as well as buoyancy, only have a small effect on the onset of instability

In this Brief Communication we study experimentally the flow regimes that appear in coaxial air-water jets discharging into a stagnant air atmosphere and we propose a simple explanation for their occurrence based on linear, local, spatiotemporal stability theory. In addition to the existence of a periodic bubbling regime for low enough values of the water-to-air velocity ratio, u = u w / u a , our experiments revealed the presence of a jetting regime for velocity ratios higher than a critical one, u c . In the bubbling regime, bubbles form periodically from the tip of an air ligament whose length increases with u. However, when u Ͼ u c a long, slender gas jet is observed inside the core of the liquid coflow. Since in the jetting regime the downstream variation of the flow field is slow, we performed a local, linear spatiotemporal stability analysis with uniform velocity profiles to model the flow field of the air-water jet. Similar to the transition from dripping to jetting in capillary liquid jets, the analysis shows that the change from the bubbling to the jetting regime can be understood in terms of the transition from an absolute to a convective instability.A common mechanism to control the generation of gas bubbles is the use of a liquid coflow surrounding the gas injection needle, a configuration that produces bubbles smaller than in the case without coflow. 1,2 In this context, one of the most relevant parameters controlling the bubble size, or the bubble formation frequency, is the velocity ratio between the liquid coflow and the gas stream at the exit of the injection needle. For sufficiently small values of the liquid-to-gas velocity ratio, u = u w / u a , the formation of bubbles is a periodic process characterized by the nonlinear growth and collapse of bubbles inside the liquid jet. However, in this Brief Communication we report experimental evidence of the transition from the aforementioned periodic bubbling regime to an aperiodic jetting regime, characterized by the formation of a long ligament of air inside the liquid jet, which occurs when the velocity ratio becomes larger than a critical value, u c . This phenomenon is similar to the transition from dripping to jetting in free liquid jets, 3,4 where the formation of a jet from the nozzle is only possible for values of the Weber number higher than a critical one. Moreover, through the use of the concepts of locally convective and absolute instabilities, applied to a simplified flow model, we show that the jetting phenomenon is related to a convective instability, while the bubbling regime is the consequence of a transition to an absolute instability. Similar flow configurations have been previously studied from different point of views. 5-8 However these works focused mainly on the production of liquid shells in compound jets where the diameter of the outer jet is nearly the same as that of the inner one. In the present study, the air-water coaxial jet discharges into a stagnant air atmosphere avoiding, therefore, any effect of the outer shear la...