Application of Micro-Schlieren Technique to Quantitative Analysis of Mass Transport in a Micromixer

In this study, we develop a micro-schlieren analysis methodology to facilitate measurement of inhomogeneities in a micromixer. As a first step, calibration procedures are performed by mixing dilute aqueous methanol and water in a T-microchannel in order to obtain the relation between the refractive index gradients and pixel grayscale values. Knowing the influences of concentration in the refractive index of aqueous methanol solution, we are able to associate pixel grayscale values with the concentration gradient along the cross-stream axis. For refractive index gradient larger than 130 m⁻¹, the calibration curve shows great linearity, setting the sensitivity limit of our quantitative micro-schlieren system to 2 × 10⁴ m⁻¹ for concentration gradient. We then implement this methodology to examine the mixing behavior in a microfluidic oscillator. The results prove that the micro-schlieren technique is able to provide spatially-resolved, noninvasive, full-field transient measurements and offer a valuable insight into the microscale mass transport phenomena that other methods fail to deliver. While the micro-schlieren technique is directly linked to the measurement of refractive index gradient, the present methodology can be extended to other scalar quantitations that are related to the variation of refractive index in the future.