A multi-port liquid bridge (1) adds aqueous phase droplets (10) in an enveloping oil phase carrier liquid (11) to a draft channel (4, 6). A chamber (3) links four ports, and it is permanently full of oil (11) when in use. Oil phase is fed in a draft flow from an inlet port (4) and exits through a draft exit port (6) and a compensating flow port (7). The oil carrier and the sample droplets (3) ("aqueous phase") flow through the inlet port (5) with an equivalent fluid flow subtracted through the compensating port (7). The ports of the bridge (1) are formed by the ends of capillaries held in position in plastics housings. The phases are density matched to create an environment where gravitational forces are negligible. This results in droplets (10) adopting spherical forms when suspended from capillary tube tips. Furthermore, the equality of mass flow is equal to the equality of volume flow. The phase of the inlet flow (from the droplet inlet port (5) and the draft inlet port (4) is used to determine the outlet port (6) flow phase.