Abstract: A microfluidic system comprises a microchannel, a carrier fluid in the microchannel, and at least two plugs in the microchannel. Each plug comprises a plug fluid that is substantially immiscible with the carrier fluid. The microfluidic system further comprises at least one spacer in the microchannel between two plugs. Each spacer comprises a spacer fluid that is substantially immiscible with the carrier fluid and the plug fluid, and both of the following conditions are satisfied: (?c-r+?t-r>?c-t) and (?c-t+yt-r>yc-r), where ?c-r is the interfacial force between the carrier fluid and the plug fluid, ?t-r is the interfacial force between the spacer fluid and the plug fluid, and ?c-t is the interfacial force between the carrier fluid and the spacer fluid.