A microfluidic device and method of its use. A workable microfluidic device (100) includes a channel (116) extending between an entry aperture (230) and a vent (118). Part of the channel (116) is arranged to permit interrogation of particles carried by a fluid passing therethrough. Suction structure carried on the device (100) is disposed in fluid communication with the channel operably to apply a reduced pressure at the entry aperture (230) effective to infuse an amount of fluid through the entry aperture and into a receiving zone (122, 122′, 122″) of the channel. The entry aperture (230) is placed into a bulk fluid, and the suction structure is operated to load a sample into the microfluidic device (100).