Figure 1.

Overview of basic microfluidic functions. a) Laminar flow at small length scales in microfluidic channels follow predictable, controllable trajectories that can be utilized to form controlled fluid boundaries. Two miscible liquids (blue and red) injected at various flow rates (Q1 and Q2) into a Y-shaped microfluidic channel can form stable boundaries that are disrupted only by diffusive mixing. b) Controlled flow boundaries and diffusive mixing can be employed to generate stable gradients. In this case, input concentrations at a few inlets can be passively mixed sequentially into multiple intermediate output concentrations⁸. Reprinted with permission. Copyright 2000 American Chemical Society. c) The flow of two immiscible fluids (usually an aqueous disperse phase and an oil-based continuous phase) can be used to form droplets of uniform size⁹. Applications include encapsulation and culture of cells and microorganisms, high-throughput parallel chemical reactions and generation chemical compound libraries for screens and subsequent processing. Reprinted with permission. Copyright 2009 American Chemical Society. d) Microfluidic fabrication with soft polymers such as polydimethylsiloxane (PDMS) can include active structures based on the deflection of thin membranes using pneumatic pressure. Applications include flow, cell and microorganism control, mechanical stimulation, fluid displacement, and pumping.