Metrology of confined flows using wide field nanoparticle velocimetry
Résumé
The manipulation of fluids in micro/nanofabricated systems opens new avenues to engineer the transport of matter at the molecular level. Yet the number of methods for the in situ characterization of fluid flows in shallow channels is limited. Here we establish a simple method called nanoparticle velocimetry distribution analysis (NVDA) that relies on wide field microscopy to measure the flow rate and channel height based on the fitting of particle velocity distributions along and across the flow direction. NVDA is validated by simulations, showing errors in velocity and height determination of less than 1% and 8% respectively, as well as with experiments, in which we monitor the behavior of 200 nm nanoparticles conveyed in channels of ~1.8 μ m in height. We then show the relevance of this assay for the characterization of flows in bulging channels, and prove its suitability to characterize the concentration of particles across the channel height in the context of visco-elastic focusing. Our method for rapid and quantitative flow characterization has therefore a broad spectrum of applications in micro/nanofluidics, and a strong potential for the optimization of Lab-on-Chips modules in which engineering of confined transport is necessary. Nanotechnologies enable the monitoring of steric, electric, and hydrodynamic interactions in confined flows, and open up new horizons in the engineering of new molecular biology assays 1 , the enhancement of osmotic energy conversion 2 , or the development of new fluid functions, including among others flu-idic diodes 3. Accurate flow characterization is most commonly carried out using image-based particle velocimetry 4. This method, which is based on wide field observation of seeded tracers, has gained popularity due to the simplicity of its implementation 5–7. It also comes with intrinsic limitations, in particular associated to the thickness of optical sections that makes it difficult to resolve flow profiles in shallow channels of less than ~1–2 μ m 8. Nanovelocimetry techniques using total internal reflection fluorescence (TIRF) imaging, which were pioneered by the groups of Yoda 9–13 and Breuer 14–17 , take advantage of the exponential decay of excitation energy to determine particle in-axis position. The prevalence of diffusive effects for small tracers, which tend to blur the flow profile, has been successfully addressed by optimizing camera frame rates and developing dedicated image treatment procedures. These technologies now allow for accurate characterizations of near-wall fluid flows, but the thin section of TIRF microscopes of ~200 nm is not adequate to monitor bulk fluid properties in confined flows. Here, we set out to develop an alternative method for accurate in situ characterization of confined flows, which merely requires a wide-field fluorescence microscope and conventional 2D tracking algorithms. Flow properties have been characterized based on the analysis of tracer velocity distributions, following a method described in the first section of this report. This method, called Nanoparticle Velocity Distribution Analysis (NVDA) in the article, is then validated using Brownian dynamics simulations of particles flowing in shallow channels. We then show the relevance of NVDA to characterize Newtonian fluid flows in 1.8 μ m thick channels, and apply it both to monitor the deformation of shallow channels associated to pressure-driven flows, and to characterize the distribution of particles across the channel height in the context of visco-elastic focusing. In a shallow channel of 2 μ m or less, tracers tracked by wide field fluorescence microscopy remain in focus throughout their migration (Fig. 1A). Their 2D trajectories can be tracked by common tracking algorithms in order to retrieve the longitudinal and lateral velocity distributions along the x-and y-axis respectively (Fig. 1B), which are used as inputs for our analysis. The longitudinal velocity distribution
Domaines
Biophysique [physics.bio-ph]
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