Reactions of biomolecules with surface mounted materials on microscope slides are often limited by slow diffusion kinetics, especially in low volumes where diffusion is the only means of mixing. This is a particular problem for reactions where only small amounts of analyte are available and the required reaction volume limits the analyte concentration. A low volume microfluidic device consisting of two interconnected 9 mm x 37.5 mm reaction chambers was developed to allow mixing and closed loop fluidic circulation over most of the surface of a microscope slide. Fluid samples are moved from one reaction chamber to the other by the rotation of a magnetic stirring bar that is driven by a standard magnetic stirrer. We demonstrate that circulation and mixing of different reagents can be efficiently accomplished by this closed loop device with solutions varying in viscosity from 1 to 16.2 centipoise. We also show by example of a microarray hybridization that the reaction efficiency can be enhanced 2-5 fold through fluid mixing under conditions where diffusion is rate limiting. For comparison, similar results were achieved with a disposable commercial device that covers only half of the reaction area of the closed loop device.