The present invention provides an apparatus comprising a substrate and first and second disks. The disks are rotatably located over the substrate, each disk having an outer circumference with teeth thereon. The first disk is positioned to interleave one or more of its teeth with the teeth of the second disk. The substrate includes a channel with an exit port located near the teeth of one of the disks. Another apparatus comprises at least one disk rotatably located over a substrate and in a well of the substrate, the disk having an outer circumference with teeth thereon. The disk is positioned to provide a maximum distance of less than about 10 microns between each one the teeth and a nearest wall defining the well. The substrate includes a channel with an exit port located near the teeth of the disk.