FIG. 1: Device schematic. The microfluidic device (a) consists of four-reservoirs (two input reservoirs at positive voltage that flow into the grounded fourth reservoir, the third reservoir floats during experiments described here) connected via microchannels. A dyed (reservoir 1) stream merges with an undyed (reservoir 2) stream and flows towards the ground (reservoir 4) through microchannels defined by a 20 µ m deep chemical etch which results in the distinctive D-scooped profile associated with wet etching. Past the intersection that merges the two input streams 200:100 µ m wide channel constrictions occur creating extensional flow fields. The small (not to scale) circle in (b) indicates the photomultiplier measurement spot ( ≈ 10 µ m focused waist of a 532 nm diode laser) used to measure fluorescence emitted by excited dye, while the large dashed circle outlines the field of view for microphotographs. We found that use of a LMW sample from Polysciences Inc. which contained ≈ 1% ammonium chloride stopped electro-osmotic flow, even after dialysis. As PAAm is known not to adhere to channels [19], and ≈ 1% ammonium chloride solutions in water:methanol solvent demonstrated normal flow behavior, the mechanism preventing flow in the sample which included salt is not known. One possibility is the presence of a contaminant copolymer similar to poly-N-hydroxyethylacrylamide [20], which adheres to side-walls and significantly reduces the electro-osmotic mobility. 5