Droplet formation in a microfluidic T-junction device which is high aspect ratio rectangular channel connected By a perpendicular channel were investigated experimentally. This geometry is quite similar to the classic T-junction device, however the perpendicular channel is a slightly narrower with respect to the dispersed phase inlet, leading to remarkably different result. The perfectly controllable droplets were found to be monodispersed with a less than 2% variation in micron size. Experimental results, including the relation between diameter and flow rates, the change of the velocity and pressure at drop break-up process, had been analyzed in detail. The single breakup process and the quasistatic character were described by evolution of the width and length of the liquid-liquid interface. Finally, in contrast to the capillary instability in an unbounded fluid, the breakup process was explained in term of absolute instabilities.
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