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Mapping the salinity gradient in a microfluidic device with schlieren imaging.

Sun CL, Chen ST, Hsiao PJ - Sensors (Basel) (2015)

Bottom Line: Since salinity variation leads to change in refractive index, the fluid mixing of an aqueous salt solution of a known concentration and water in a T-microchannel is used to establish the relation between salinity gradients and grayscale readouts.For saline solution with salinity close to that of the seawater, the grayscale readouts vary linearly with the salinity gradient, and the regression line is independent of the flow condition and the salinity of the injected solution.It is shown that the schlieren technique is well suited to quantify the salinity gradients in microfluidic devices, for it provides a spatially resolved, non-invasive, full-field measurement.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical Engineering, National Taiwan University, 1 Roosevelt Road Section 4, Taipei 10617, Taiwan. clsun@ntu.edu.tw.

ABSTRACT
This work presents the use of the schlieren imaging to quantify the salinity gradients in a microfluidic device. By partially blocking the back focal plane of the objective lens, the schlieren microscope produces an image with patterns that correspond to spatial derivative of refractive index in the specimen. Since salinity variation leads to change in refractive index, the fluid mixing of an aqueous salt solution of a known concentration and water in a T-microchannel is used to establish the relation between salinity gradients and grayscale readouts. This relation is then employed to map the salinity gradients in the target microfluidic device from the grayscale readouts of the corresponding micro-schlieren image. For saline solution with salinity close to that of the seawater, the grayscale readouts vary linearly with the salinity gradient, and the regression line is independent of the flow condition and the salinity of the injected solution. It is shown that the schlieren technique is well suited to quantify the salinity gradients in microfluidic devices, for it provides a spatially resolved, non-invasive, full-field measurement.

No MeSH data available.


Related in: MedlinePlus

(a) Distributions of salinity gradient (numerical simulation); (b) Grayscale ratios (micro-schlieren images) in the T-microchannel, Re = 5.
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sensors-15-11587-f002: (a) Distributions of salinity gradient (numerical simulation); (b) Grayscale ratios (micro-schlieren images) in the T-microchannel, Re = 5.

Mentions: The micro-schlieren images with the distributions of salinity gradient in the T-microchannel from simulation for Re = 5 and S = 40 g·kg−1 are compared (Figure 2). In the left panel (Figure 2), the salinity solution is injected into the T-microchannel from the upper inlet, while the opposite is shown in the right panel (Figure 2). In this study, the knife-edge is placed such as to obstruct the light bending toward the +y direction. Since the refractive index of the saline solution is higher than that of water, light deflects in the direction of increasing salinity. As a result, positive salinity gradient leads to a dark band, whereas negative salinity gradient produces a light band. The broadening band along the streamwise direction is ascribed to the dispersion effect. In the downstream region, salinity gradient diminishes and the schlieren band blurs. Although the distributions of salinity gradient and the micro-schlieren images exhibit great similarity, the simulation predicts a weaker dispersion effect than the experiments. Two factors may contribute to this deviation: interdiffusion between different species presented in our saline solution and the variation of diffusion coefficient with concentration. On one hand, ionic interactions lead to modification of the fluxes of diffusing ions and affect the relative distribution. When the mixing interface establishes in the T-microchannel, gradients of MgCl2 and Na2SO4 produce electric field that drives the fluxes of the ample Na+ and Cl− ions, resulting in an enhancement of dispersion [42]. On the other hand, the diffusion coefficient of NaCl increases 4.1% as salinity reduces from 40 g·kg−1 to 0 [40], promoting diffusion near the edge of the band. Indeed, the simulation model can be improved by considering the multi-component diffusion. Nevertheless, it remains difficult to account for the contribution of different constituents to the change in local refractive index. For the purpose of detecting salinity gradient, we decide to treat seawater as a first-order composition system in this study [43].


Mapping the salinity gradient in a microfluidic device with schlieren imaging.

Sun CL, Chen ST, Hsiao PJ - Sensors (Basel) (2015)

(a) Distributions of salinity gradient (numerical simulation); (b) Grayscale ratios (micro-schlieren images) in the T-microchannel, Re = 5.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4482010&req=5

sensors-15-11587-f002: (a) Distributions of salinity gradient (numerical simulation); (b) Grayscale ratios (micro-schlieren images) in the T-microchannel, Re = 5.
Mentions: The micro-schlieren images with the distributions of salinity gradient in the T-microchannel from simulation for Re = 5 and S = 40 g·kg−1 are compared (Figure 2). In the left panel (Figure 2), the salinity solution is injected into the T-microchannel from the upper inlet, while the opposite is shown in the right panel (Figure 2). In this study, the knife-edge is placed such as to obstruct the light bending toward the +y direction. Since the refractive index of the saline solution is higher than that of water, light deflects in the direction of increasing salinity. As a result, positive salinity gradient leads to a dark band, whereas negative salinity gradient produces a light band. The broadening band along the streamwise direction is ascribed to the dispersion effect. In the downstream region, salinity gradient diminishes and the schlieren band blurs. Although the distributions of salinity gradient and the micro-schlieren images exhibit great similarity, the simulation predicts a weaker dispersion effect than the experiments. Two factors may contribute to this deviation: interdiffusion between different species presented in our saline solution and the variation of diffusion coefficient with concentration. On one hand, ionic interactions lead to modification of the fluxes of diffusing ions and affect the relative distribution. When the mixing interface establishes in the T-microchannel, gradients of MgCl2 and Na2SO4 produce electric field that drives the fluxes of the ample Na+ and Cl− ions, resulting in an enhancement of dispersion [42]. On the other hand, the diffusion coefficient of NaCl increases 4.1% as salinity reduces from 40 g·kg−1 to 0 [40], promoting diffusion near the edge of the band. Indeed, the simulation model can be improved by considering the multi-component diffusion. Nevertheless, it remains difficult to account for the contribution of different constituents to the change in local refractive index. For the purpose of detecting salinity gradient, we decide to treat seawater as a first-order composition system in this study [43].

Bottom Line: Since salinity variation leads to change in refractive index, the fluid mixing of an aqueous salt solution of a known concentration and water in a T-microchannel is used to establish the relation between salinity gradients and grayscale readouts.For saline solution with salinity close to that of the seawater, the grayscale readouts vary linearly with the salinity gradient, and the regression line is independent of the flow condition and the salinity of the injected solution.It is shown that the schlieren technique is well suited to quantify the salinity gradients in microfluidic devices, for it provides a spatially resolved, non-invasive, full-field measurement.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical Engineering, National Taiwan University, 1 Roosevelt Road Section 4, Taipei 10617, Taiwan. clsun@ntu.edu.tw.

ABSTRACT
This work presents the use of the schlieren imaging to quantify the salinity gradients in a microfluidic device. By partially blocking the back focal plane of the objective lens, the schlieren microscope produces an image with patterns that correspond to spatial derivative of refractive index in the specimen. Since salinity variation leads to change in refractive index, the fluid mixing of an aqueous salt solution of a known concentration and water in a T-microchannel is used to establish the relation between salinity gradients and grayscale readouts. This relation is then employed to map the salinity gradients in the target microfluidic device from the grayscale readouts of the corresponding micro-schlieren image. For saline solution with salinity close to that of the seawater, the grayscale readouts vary linearly with the salinity gradient, and the regression line is independent of the flow condition and the salinity of the injected solution. It is shown that the schlieren technique is well suited to quantify the salinity gradients in microfluidic devices, for it provides a spatially resolved, non-invasive, full-field measurement.

No MeSH data available.


Related in: MedlinePlus