Controlled formation of double-emulsion drops in sudden expansion channels. 
Double-emulsion drops or drops-in-drop have provided useful templates for production of microcapsules due to their core-shell geometry.  Here, we introduce new capillary microfluidic geometry for the creation of double-emulsion drops, which is composed of a narrowing channel followed by sudden expansion channel.  Drops injected through the narrowing channel are highly accelerated to flow, inducing high inertia force.  When rear interface of the drops arrives at the sudden expansion channel, the high inertia force deforms the interface and leads to its breakup into a drop in the interior of the injected drop.  This insertion is driven by inertia force against capillary force: High linear velocity and low interfacial tension facilitate the insertion.  We also apply this emulsification method to double-emulsion drops with single innermost drop; insertion of a water drop creates the double-emulsion drops with two distinct innermost drops.  The resultant double-emulsion drops with single- or double-innermost drops provide useful templates to produce polymersomes which encapsulate same fluid to the continuous phase; this will be potentially useful for sampling of the continuous phase and its isolation in a wide range of applications for micro-total analysis system (μ-TAS).