of the first channel segment; selectively apply a second differential driving force to the second channel segment that is sufficient to substantially eliminate a differential driving force between the first end of the first channel segment and the first fluid junction; and selectively apply a third differential driving force through the third channel segment sufficient to substantially eliminate a differential driving force between the second fluid junction and the second end of the first channel segment.

25. The system of claim 24, wherein the first, second and third channels are disposed in a single integrated body structure.

26. The system of claim 24, wherein the flow controller comprises a pressure source operably coupled to at least one of the first and second ends of the first chaimel segment.

27. The systemand second capillary elements fluidly coupled to the first channel segments, the first and second fluid junctions being disposed along the first channel segment at points between points at which the first and second capillary elements are in fluid communication with the first channel segment, at least one of the first and second capillary elements being an input pipettor.

33. The system of claim 24, further comprising an input pipettor and an output nozzle, the input pipettor being fluidly coupled to the first end of the first channel segment and the output nozzle being fluidly coupled to the second end of the first channel segment.

34. A method of sampling and dispensing materials, comprising: providing a microfluidic device that comprises: a first channel network comprising at least one valve module,