A method for forming and collecting a plurality of plugs, the method comprising: providing a microfluidic device comprising a substrate having at least one microchannel formed therein, the at least one microchannel having an inlet, an outlet downstream from the inlet, and a variable cross-section along its length; introducing a stream of aqueous fluid into the inlet of the microchannel and flowing, in response to application of pressure upon the microfluidic device, the stream of aqueous fluid through the microchannel downstream towards the outlet of the microchannel, wherein the aqueous fluid contains a mixture of at least one DNA or RNA molecule and one or more reagents for conducting a reaction with the at least one DNA or RNA molecule; providing a carrier fluid within the substrate and at the outlet of the microchannel, wherein the carrier fluid is immiscible with the aqueous fluid; forming a plurality of plugs of the aqueous fluid in the carrier fluid based, at least in part, on the flow of the stream of aqueous fluid and surface tension of the aqueous fluid relative to the carrier fluid at the outlet of the microchannel, wherein a subset of the plurality of plugs comprises the at least one DNA or RNA molecule and the one or more reagents for conducting the reaction with the at least one DNA or RNA molecule, and wherein the cross-section of a plug of the plurality of plugs is substantially the same size as the cross-section of the microchannel at the plug-forming region; and conducting the reaction on one or more of the plurality of plugs on the substrate to form a reaction product, wherein each of the plurality of plugs is substantially surrounded by the carrier fluid and characterised in that the interface between the plugs and the carrier fluid comprises a fluorosurfactant terminated with an oligoethylene glycol group. providing a microfluidic device comprising a substrate having at least one microchannel formed therein, the at least one microchannel having an inlet, an outlet downstream from the inlet, and a variable cross-section along its length; introducing a stream of aqueous fluid into the inlet of the microchannel and flowing, in response to application of pressure upon the microfluidic device, the stream of aqueous fluid through the microchannel downstream towards the outlet of the microchannel, wherein the aqueous fluid contains a mixture of at least one DNA or RNA molecule and one or more reagents for conducting a reaction with the at least one DNA or RNA molecule; providing a carrier fluid within the substrate and at the outlet of the microchannel, wherein the carrier fluid is immiscible with the aqueous fluid; forming a plurality of plugs of the aqueous fluid in the carrier fluid based, at least in part, on the flow of the stream of aqueous fluid and surface tension of the aqueous fluid relative to the carrier fluid at the outlet of the microchannel, wherein a subset of the plurality of plugs comprises the at least one DNA or RNA molecule and the one or more reagents for conducting the reaction with the at least one DNA or RNA molecule, and wherein the cross-section of a plug of the plurality of plugs is substantially the same size as the cross-section of the microchannel at the plug-forming region; and conducting the reaction on one or more of the plurality of plugs on the substrate to form a reaction product, wherein each of the plurality of plugs is substantially surrounded by the carrier fluid and characterised in that the interface between the plugs and the carrier fluid comprises a fluorosurfactant terminated with an oligoethylene glycol group.