The present invention relates generally to microfluidic systems and, more specifically, to apparatuses and methods associated with mixing in microfluidic systems. In some embodiments, a mixer is constructed and arranged to mix at least a portion of a first and a second fluid component. The mixer may include a channel having an inlet that separates into at least two branches, the branches then recombining into a single outlet. In some cases, plugs of fluid (e.g., a gas) are flowed into the branches, which causes changes in resistance, and thus the amount of fluid flow, in each of the branches. The motion of the plugs through the network of branched channels can create unsteady mixing flows. For instance, for two fluid components, e.g., two streams of fluid flowing laminarly in the channel, these changes in resistance can cause the crossing of laminar streamlines of the fluid, which can lead to exponential stretching and folding of the interface between the two unmixed streams.
FEDERALLY SPONSORED RESEARCH
 Various aspects of the present invention were sponsored by the U.S. Department of Energy under award DE-FG02-OOER45852 and in part by the National Institute of Health (NIGMS) under award GM065364. The Government may have certain rights in the invention.