Many parallel biochemical assays rely on thin aqueous films to spread a reactant solution over a wide area decorated with multiple distinct substrates. In this asymmetric, microfluidic geometry, diffusion limits the transport of reactants to substrates. Chemical equilibrium, a requirement for reproducibility of results, can take days to achieve.

The liquid-on-liquid mixing (LOLM) method overcomes the diffusion barrier by layering an immiscible spectator fluid, such as mineral oil, on the thin film. Stirring the spectator fluid transmits shear at the liquid-liquid interface into the thin film. The mixing accelerates the march towards equilibrium. This technique increases the speed and sensitivity of immunofluorescence staining of Drosophila larval polytene chromosomes by a factor of 100 in time and concentration, when compared to standard coverslip techniques.

Flow visualization experiments reveal the fluid motions in the thin aqueous layer. Using time-lapse video photography to monitor the evolution of a drop of colloidal dye in the thin film, I estimate the time needed to achieve good mixing at various stir rates.

The major aim of this technique is improving the hybridization step in DNA microarrays. I printed microarrays and subjected them to hybridizations with varying stir rates, durations, and target DNA concentrations. My data suggest that the mixing produces at best a modest improvement in efficiency, uniformity, sensitivity, and specificity when compared to microarrays incubated with the standard coverslip method.