Bubble production by air filament and cavity breakup in plunging breaking wave crests

Air filaments and cavities in plunging breaking waves, generically cylinders, produce bubbles through an interface instability. The effects of gravity, surface tension and surface curvature on cylinder breakup are explored. A generalized dispersion relation is obtained that spans the Rayleigh–Taylor and Plateau–Rayleigh instabilities as cylinder radius varies. The analysis provides insight into the role of surface tension in the formation of bubbles from filaments and cavities. Small filaments break up into bubbles through a Plateau–Rayleigh instability driven through the action of surface tension. Large air cavities produce bubbles through a Rayleigh–Taylor instability driven by gravity and moderated by surface tension, which has a stabilizing effect. Surface tension, interface curvature and gravity are all important for cases between these two extremes. Predicted unstable mode wavenumber and bubble size show good agreement with direct numerical simulations of plunging breaking waves and air cylinders.

Biofuel Emulsifier Using High-Velocity Impinging Flows and Singularities in Microchannels

The objective of this experimental work is to design an original microfluidic mixer for continuous emulsification of small fractions of water in a lipid phase. This system is aimed to be integrated on-line in the process so as to avoid the use of a surfactant. The currently targeted application is a better combustion of water-supplemented alternative biofuels in boilers, turbines, or internal combustion engines in general. Therefore, mean size of droplets of water in the emulsion should be 5–10 μm, and the water content should not exceed ∼20%. Microsystems developed in this work are designed so as to enhance different flow perturbations that are favorable for the emulsification process. The microchannels for the fluids admittance have different sections: 300 × 300 μm2 and 600 × 600 μm2. As a consequence, an impinging flow is developed at the crossing of the inlet microchannels of the two phases which has for effect a significant stretching of the fluids. Then, depending on the continuous phase, Rayleigh instabilities can be developed in the straight parts of the outlet channels (600 × 600 μm2) and/or the enhancement of fluid splitting is obtained; thanks to a singularity (bend) located in the same outlet channels. Two different continuous phases are tested (gasoil and sun flower oil) for which the flow rate is about (65–100 ml/min). The water fraction is varied in the range 7–24%. It is shown that the length of the outlet microchannels is a crucial parameter. Considering an oil phase with low viscosity, such as gasoil, a too long channel can promote coalescence. On the opposite, longer outlet channels are needed with more viscous fluids (like sunflower oil) in order to develop Rayleigh instabilities which is, in this case, the more efficient way to obtain emulsions in this kind of microsystem. On a general point of view, concerning the size of the water droplets, dispersion of water is much more efficient with this microsystem using gasoil rather than vegetable oil as the continuous phase. Considering the targeted application, emulsions with an average size of water droplets of about 10 μm were obtained with gasoil as the continuous phase.

Temperature-triggered shape-transformations in layer-by-layer microtubes

Freely released microtubes in water transform to ellipsoids and spheres at high temperatures, while microtubes bound to the template surface showed periodic voids suggestive of Rayleigh instabilities.