2 results
Near-wall nanovelocimetry based on total internal reflection fluorescence with continuous tracking
- Zhenzhen Li, Loïc D’eramo, Choongyeop Lee, Fabrice Monti, Marc Yonger, Patrick Tabeling, Benjamin Chollet, Bruno Bresson, Yvette Tran
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- Journal:
- Journal of Fluid Mechanics / Volume 766 / 10 March 2015
- Published online by Cambridge University Press:
- 02 February 2015, pp. 147-171
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The goal of this work is to make progress in the domain of near-wall velocimetry. The technique we use is based on the tracking of nanoparticles in an evanescent field, close to a wall, a technique called TIRF (total internal reflection fluorescence)-based velocimetry. The particles are filmed continuously, with no time gap between two frames, so that no information on their trajectories is lost. A number of biases affect the measurements: Brownian motion, heterogeneities induced by the walls, statistical biases, photobleaching, polydispersivity and limited depth of field. Their impacts are quantified by carrying out Langevin stochastic simulations, in a way similar to Guasto & Breuer (Exp. Fluids, vol. 47, 2009, pp. 1059–1066). By using parameters calibrated separately or known, we obtain satisfactory agreement between experiments and simulations, concerning the intensity density distributions, velocity fluctuation distributions and the slopes of the linear velocity profiles. Slip lengths measurements, taken as benchmarks for analysing the performances of the technique, are carried out by extrapolating the corrected velocity profiles down to the origin along with determining the wall position with an unprecedented accuracy. For hydrophilic surfaces, we obtain $1\pm 5~\text{nm}$ for the slip length in sucrose solutions and $9\pm 10~\text{nm}$ in water, and for hydrophobic surfaces, $32\pm 5~\text{nm}$ for sucrose solutions and $55\pm 9~\text{nm}$ for water. The errors (based on 95 % confidence intervals) are significantly smaller than the state of the art, but more importantly, the method demonstrates for the first time a capacity to measure slippage with a satisfactory accuracy, while providing a local information on the flow structure with a nanometric spatial precision and velocity errors of a few per cent. Our study confirms the discrepancy already pointed out in the literature between numerical and experimental slip length estimates. With the progress conveyed by the present work, TIRF-based technique with continuous tracking can be considered as a quantitative method for investigating flow properties close to walls, providing both global and local information on the flow.
Experimental study and nonlinear dynamic analysis of time-periodic micro chaotic mixers
- YI-KUEN LEE, CHIANG SHIH, PATRICK TABELING, CHIH-MING HO
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- Journal:
- Journal of Fluid Mechanics / Volume 575 / March 2007
- Published online by Cambridge University Press:
- 07 March 2007, pp. 425-448
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The efficiency of MEMS-based time-periodic micro chaotic mixers is experimentally and theoretically investigated in this study. A time-periodic flow perturbation was realized using digitally controlled solenoid valves to activate a source and sink alternately, acting together as a pair, with different driving frequencies. Working fluids with and without fluorescent dye were used in the micromixing experiments. The spatio-temporal variation of the mixing concentration during the mixing process was characterized at different Strouhal numbers, ranging from 0.03 to 0.74, using fluorescence microscopy. A simple kinematical model for the micromixer was used to demonstrate the presence of chaotic mixing. Specific stretching rate, Lyapunov exponent, and local bifurcation and Poincaré section analyses were used to identify the emergence of chaos. Two different numerical methods were employed to verify that the maximum Lyapunov exponent was positive in the proposed micromixer model. A simplified analytical analysis of the effect of Strouhal number is presented. Kolmogorov–Arnold–Mose (KAM) curves, which are mixing barriers, were also found in Poincaré sections. From a comparative study of the experimental results and theoretical analysis, a finite-time Lyapunov exponent (FTLE) was shown to be a more practical mixing index than the classical Lyapunov exponent because the time spent in mixing is the main concern in practical applications, such as bio-medical diagnosis. In addition, the FTLE takes into account both fluid stretching in terms of the stretching rate and fluid folding in terms of curvature.