Abstract: A microfluidic device may include a microstructure formed in a substrate, the microstructure including a primary channel with a first end and a second end, and a plurality of chambers that open to the primary channel. At least two openings coupled to the first end of the primary channel may be used to load at least two fluid streams into the device through the first end of the primary channel to flow along the primary channel from the first end to the second end into the plurality of chambers, each chamber of the plurality of chambers having a volume less than 100 nanoliters and connected by a vent to a secondary channel in the micro structure, a width of the vent configured to enable a gas to escape from the chamber to the secondary channel while inhibiting the flow of said at least first and second fluid streams into the secondary channel.