Imaging diffusion in a microfluidic device by third harmonic microscopy
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We monitor and characterize near-surface diffusion of miscible, transparent liquids in a microfluidic device by third harmonic microscopy. The technique enables observations even of transparent or index-matched media without perturbation of the sample. In particular, we image concentrations of ethanol diffusing in water and estimate the diffusion coefficient from the third harmonic images. We obtain a diffusion coefficient D = (460 ± 30) μm2/s, which is consistent with theoretical predictions. The investigations clearly demonstrate the potential of harmonic microscopy also under the challenging conditions of transparent fluids.