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Microfluidic devices are widely used in microbiology, fine organic synthesis, pharmaceuticals, biomedicine, etc. Most applications require rapid mixing of the fluids that pass through the microfluidic chip. Continuous-flow microreactors used in flow chemistry have a characteristic channel size that is small enough, compared to standard laboratory size in fluid mechanics but large enough to render the diffusion mixing mechanism ineffective. In this work, we study, experimentally and theoretically, the efficiency of using various mechanisms of natural convection for the mixing of fluids entering the microfluidic chip. Solutions typically differ in buoyancy and diffusion rates of solutes, making them sensitive to gravity-dependent instabilities such as Rayleigh-Taylor convection, double diffusion, and diffusion layer convection. We consider a Y-shaped microreactor, which is, on the one hand, the simple scheme of mixing and, on the other hand, a typical element of a microfluidic network. We assume that two miscible solutions independently enter through different tubes into a common channel, where they come into contact and begin to mix. For simplicity, we do not consider chemical reactions in this paper. For each type of instability, we numerically estimated the characteristic channel length, after which complete mixing of the solutions occurs. The numerical simulations are performed in the framework of the 3D model. Finally, we compare the experimental data and numerical results. [ABSTRACT FROM AUTHOR]

An experimental investigation of chemically triggered convection is carried out. A two-layer system of miscible reactive fluids—the neutralization reaction between an acid and a base—is considered in a vertical Hele-Shaw cell. This system is subject to the action of gravity and vertical translational vibrations (modulated gravity). During the reaction, a less dense salt solution emerges in the reaction zone, thus triggering the buoyancy-driven convection in the form of concentration plumes rising in the upper layer. The particle image velocimetry is used as the method of study. The considered system is non-stationary, and a technique is developed for the processing of a long series of images registered during experiments. The evolution of the flow structures and of the convection intensity is analyzed. In the considered range of experimental parameters the vibrations slightly slow down the convection. [ABSTRACT FROM AUTHOR]

In this work, we focus on the processes which accompany a frontal neutralization reaction occurring between two miscible fluids filling a vertical Hele-Shaw cell. We have found that chemically-induced changes of reagent concentrations coupled with concentration- dependent diffusion (CDD) can produce spatially localized low density areas which are sensitive to the external inertial field. In the case of static gravity we have demonstrated both experimentally and theoretically that it can give rise to the development of perfectly periodic convective structure. This scenario is strikingly different from the irregular density fingering, which is typically observed in the miscible systems. When the system is under the influence of the periodic low-frequency vibrations perpendicular to the reaction front, we found numerically the excitation of a mixed-mode instability combining the double-diffusion instabilities and the Rayleigh-Taylor mechanism of the convection within the low density areas. [ABSTRACT FROM AUTHOR]