Mapping the Salinity Gradient in a Microfluidic Device with Schlieren Imaging

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.

Low-Cost Resistive Microfluidic Salinity Sensor for High-Precision Detection of Drinking Water Salt Levels

“… 24 Moreover, the surface to volume ratio in microfluidic devices increases substantially due to the scaling laws of physics, which can lead to attaining high levels of sensitivity and low levels of detection. Sun et al 25 used grayscale readouts from a microscope to detect salinity concentration changes in water on a microfluidic device. This method was restricted to seawater samples as changes at lower salt concentrations did not change the visual readouts.…”

Estuary-on-a-chip: unexpected results for the fate and transport of nanoparticles

“…Recently, new experimental methods and capacities, based on microfluidic devices, are opening the door to environmental science for better understand the fate of species in dynamic systems, closer to real natural conditions [15][16][17][18] . The aim of this work is to evaluate the fate of nanoparticles, using fullerene aggregates (nC 60 ), in a salinity gradient.…”

Visualization and label-free quantification of microfluidic mixing using quantitative phase imaging

“…Despite the many challenges, there is a strong motivation to provide quantitative imaging of microfluidic mixing, especially if that imaging can be achieved without introducing exogenous labeling agents. Refractive index (RI) variation in microfluidic mixing can be detected by various optical methods including schlieren microscopy, speckle photography, and surface plasmon resonance [8][9][10][11][12][13][14]. However, these techniques do not provide quantitative measurements of microfluidic concentration.…”