  We propose a novel microfluidic "opposed-flow" geometry in which the
continuous fluid phase is fed into a junction in a direction opposite the
dispersed phase. This pulls out the dispersed phase into a micron-sized jet,
which decays into micron-sized droplets. As the driving pressure is tuned to a
critical value, the jet radius vanishes as a power law down to sizes below 1
$\mu$m. By contrast, the conventional "coflowing" junction leads to a first
order jetting transition, in which the jet disappears at a finite radius of
several $\mu$m, to give way to a "dripping" state, resulting in much larger
droplets. We demonstrate the effectiveness of our method by producing the first
microfluidic silicone oil emulsions with a sub micron particle radius, and
utilize these droplets to produce colloidal clusters.
