Generation of micron-sized bubbles at the entrance region of PDMS microchannels

Here we present a new regime of operation of PDMS-based flow focusing microfluidic devices. We show that bubbles with diameters below one tenth the channel width, which we fix here to $w=50$ $\mu$m, can be produced in low viscosity liquids thanks to the strong pressure gradient existing at the entrance region of the channel. Our theory, which is in good agreement with experiments, predicts that bubble size can be expressed as $d_b/w\propto \left(Q_g/Q_g\right)^{5/12}$, where $Q_g$ and $Q_l$ indicate, respectively, the gas and liquid flow rates.