Geometry-controlled droplet generation in head-on microfluidic devices

We investigated the generation of droplets in a head-on microfluidic device operated with the two identical channels as inlets and the long leg as a constriction channel leading to a wider outlet section. For capillary numbers (Ca) of approximately 10(-5) or less, we find a Ca-independent droplet volume equal to the volume of the constriction channel, which decreases at higher Ca when shear forces become relevant. The droplet generation mechanism is explained in terms of a global capillary instability involving surprisingly stable intermediate surface configurations. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3000624]