We study the breakup of confined fluid threads at low flow rates to
understand instability mechanisms. To determine the critical conditions between
the earlier quasi-stable necking stage and the later unstable collapse stage,
simulations and experiments are designed to operate at an extremely low flow
rate. Critical mean radii at neck centres are identified by the stop-flow
method for elementary microfluidic configurations. Analytical investigations
reveal two distinct origins of capillary instabilities. One is the gradient of
capillary pressure induced by the confinements of geometry and external flow,
whereas the other is the competition between local capillary pressure and
internal pressure determined by the confinements
