Suppression of instabilities in multiphase flow by geometric confinement.

We investigate the effect of confinement on drop formation in microfluidic devices. The presence or absence of drop formation is studied for two immiscible coflowing liquids in a microfluidic channel, where the channel width is considerably larger than the channel height. We show that stability of the inner fluid thread depends on the channel geometry: when the width of the inner fluid is comparable to or larger than the channel height, hydrodynamic instabilities are suppressed, and a stable jet that does not break into drops results; otherwise, the inner fluid breaks into drops, in either a dripping or jetting regime. We present a model that accounts for the data and experimentally exploit this effect of geometric confinement to induce the breakup of a jet at a spatially defined location.