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 α (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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