FIGURE 1. Microfluidic interfaces provide unique functionality.

a, Floating interfaces between immiscible fluids can be used to produce droplets of precise shape and varying content. These can act as microscale containers with permeable walls for performing and analysing reactions, creating custom magnetic or protein-coated vesicles, or transporting cargo. b, Pinned interfaces between immiscible liquids are created by selective surface patterning of a microchannel. They can be used to create vertical interfaces between liquid and air that, defying gravity, can hold liquids without breaking, or to stabilize the interface between immiscible fluids allowing interfacial chemistry to form real walls. c, Moving interfaces between miscible liquids are created under laminar flow conditions between two streams of fluid flowing together because they do not mix except by diffusion, creating a diffusive interface with predictable geometry. d, Secondary interfaces arise in microscale channels because transport is mainly due to diffusion (convection-free environments). Diffusion can create complex but predictable patterns (interfaces) of solutes based on the diffusivity of the solutes.