(464e) Experimental Visualization of Mass Transfer in Nanofluids Using TIRF Microscopy

Nanofluids ? fluids containing suspended nanoparticles at low volume fractions ? are presented as an interesting means to improve the cooling capability of conventional heat transfer fluids. Owing to their small size, suspended nanoparticles do not show the major drawbacks associated with suspensions of micrometer or larger size solid particles (erosion, clogging and severe pressure drop). This relative advantage brought many researchers to conduct experimental and numerical investigations to evaluate the cooling capabilities of nanofluids. These investigations have shown an enhancement of the thermal conductivity and, in several cases, this enhancement is anomalously high when compared to continuum model predictions (e.g. Maxwell and Hamilton-Crosser models). However, the reported enhancements show a significant spread and more work is needed to improve our fundamental understanding of such complex systems. Of the many mechanisms that were proposed to explain the anomalous enhancement of nanofluids thermal conductivity, two have emerged of the debate: nanoparticles aggregation and Brownian motion-induced nanoparticles hydrodynamics. The latter mechanism seems more interesting since it could also explain why nanofluids greatly affect mass transport behaviour. Indeed, dye diffusion experiments in suspensions of nanoparticles in water have shown a diffusion pattern that is no longer isotropic and for which the diffusion coefficient is more than one order of magnitude larger than expected. Still, no experimental evidences supporting this suggestion of Brownian motion-induced nanoparticles hydrodynamics have been reported yet.

This project aims at visualizing and characterizing the hydrodynamics of mass transfer in nanofluids using Total Internal Reflection Fluorescence Microscopy (TIRFM). The similarities between heat and mass transfer processes will then be used to infer an explanation for the anomalous enhancement of nanofluids thermal conductivity. TIRFM is chosen over traditional fluorescence and confocal microscopy for its ability to greatly reduce the amount of undesired background fluorescence. Moreover, it probes phenomena occurring very near surfaces (within 200-250 nm), making the technique suitable to study microdevices where nanofluids are likely to find applications. To capture images, TIRFM requires the excitation of fluorescent molecules (or nanoparticles) suspended in the sample fluid by an evanescent wave. In our particular case, TIRFM relies on the fluorescence (emission) tracking of both the host liquid (solution of rhodamine 590 chloride) and the suspended nanoparticles (quantum dots) at two different wavelengths upon laser light stimulation. Filters are used to physically separate the wavelengths before image capture. This experimental setup enables the tracking of individual Brownian nanoparticles (quantum dots) independently of the dye diffusion process, thus making possible the development and validation of a mass transport hydrodynamics model of nanofluids.

In this paper, diffusion of rhodamine 590 chloride in water/alumina nanofluids of various volume fractions is first studied by TIRFM at a magnification of 100X and compared to diffusion in pure water. The injection of the dye solution is realized via a custom-designed PTFE/glass cell that ensures the experimental reproduction of a Heaviside-type initial condition for the concentration profile. TIRFM images are acquired at a rate of 1/3 frame per second, and both qualitative and quantitative analyses are performed. Experimental mass diffusivities are determined via a curve fitting of the acquired concentration versus time profiles at two locations along the PTFE/glass cell. Preliminary results show that the proposed method accurately measures the diffusion coefficient of rhodamine 590 chloride in deionized water, and that the diffusion process in water/alumina nanofluids is affected by the presence of nanoparticles. Current work is focusing on the precise measurement of the diffusion coefficient in nanofluids of various volume fractions, as nanoparticle loading/interaction is expected to influence the diffusion process.