Low volume-fraction (~1%) suspensions of solid nanoparticles (1–100 nm diameter) have been observed to exhibit remarkably enhanced heat transport properties relative to the particle-free fluid alone. These unique characteristics have generated intense interest in harnessing these so-called nanofluid suspensions for use in thermal management applications. Although most work to date has focused on enhancement of thermal properties, recent studies have also suggested that a corresponding increase in effective mass diffusivity is achievable in aqueous suspensions of 20 nm alumina (Al₂O₃) nanoparticles relative to that of pure water. Although this raises exciting possibilities for manipulation of microscale mixing, there are still insufficient data and theoretical insights to draw meaningful conclusions about the magnitude and scalability of these effects.

Here we employ a microfluidic system that allows us to investigate this phenomena by direct visualization of fluorescent tracer diffusion between nanoparticle-laden fluid streams as they travel through a microchannel network. This is accomplished using both "top view" imaging of parallel dye streams and confocal laser scanning microscopy to obtain a cross-sectional view of the flow. We investigate aqueous suspensions of Al₂O₃ nanoparticles with mean size ranging from 20 to 100 nm at concentrations ranging from 0.001 to 1 vol%. Diffusion coefficient values associated with the tracer dye were determined by measuring the growth of the mixed interfacial zone between the streams. Suspension viscosity and thermal conductivity were also characterized, and particle size was determined using dynamic light scattering. A key observation of this work is that the observed dye transport is strongly dependent on the composition of the nanoparticle suspension and that these parameters must be carefully controlled in order to draw correct conclusions about diffusivity. For example, we observe that the measured diffusivity of a food dye tracer nearly doubles as the nanoparticle concentration is increased to 0.5 vol% while a much more modest change is observed with Rhodamine B. These effects arise as a consequence of interactions among the nanoparticles, dye molecules, and stabilizing additives. Understanding the nature of these interactions has the potential to enable new ways to manipulate mass transport and mixing in microfluidic systems.