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The buoyancy-driven chemoconvection induced by a neutralization reaction is theoretically studied for a system consisting of nitric acid and sodium hydroxide aqueous solutions placed in a vertically oriented Hele-Shaw cell. This pair of reactants is a representative case of reacting miscible acid–base systems investigated experimentally in Part 1 of this work (Mizev et al., J. Fluid Mech., vol. 916, 2021, A22.). We showed that the list of the possible instabilities in this system is much richer than previously thought. A new scenario for pattern formation depends on a single parameter denoted by , then a potential well collapses, and a shock-wave-like structure with an almost planar front occurs. This wave propagates fast compared with the diffusion time and acts as a turbulent bore separating immobile fluid and an area of intense convective mixing. Finally, we determine the place of the above instabilities in an extended classification of known instability types.

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.

In this paper, the mixing process of two solutions of inorganic salts in a continuous flow channel with Y-type micromixer is investigated. Due to different diffusivity rates of solutes the double-diffusion convection develops in the channel. To visualize convective flows and the distribution of mixing substances the shear interferometer technique and a fluorescent dye are used. The latter makes it possible to quantify the mixing degree. The mixing extent for different volume flow rates is calculated. The comparison of convection and pure diffusion mechanisms of mixing is provided.

Continuous-flow microfluidic devices are applied in the study of microorganisms, in genetic research, production of pharmaceutical substances, lab-on-a-chip technology, biomedicine etc. Some applications require continuous mixing of the solutions that flow through the devices. However, straight-line mechanical mixing methods cannot be used due to the small size of the channels. In this paper, we discuss from a theoretical and experimental point of view the prospects of using various mechanisms of natural or forced convection for efficient mixing of solutions entering a microfluidic chip. Different designs of micromixers operating on gravity-dependent instabilities of the Rayleigh-Taylor type, double diffusion convection, and surface-dependent Marangoni instability are considered. Micromixers, in which the fluid flow is controlled by an electro-osmotic mechanism and directional deformations of the channel walls, are considered as examples of forced convection. For each case, we will provide the assessment of the range of chip sizes in which this mixing mechanism works effectively. The examples of experimental implementation of different mixing principles are given.

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.

This paper focuses on experimental investigations of the spatio-temporal distributions of fluid velocity and temperature and the concentration of reagents and reaction products. We study concentration-dependent diffusion (CDD) convection driven by the neutralization reaction in a two-layer miscible system in a vertical Hele-Shaw cell using the original experimental complex. A comprehensive understanding of the physical mechanisms of convective motion and instabilities requires employing various experimental methods simultaneously. The proposed experimental complex provides simultaneous visualization and facilitates identification of the location of the reaction front, which is of importance to the study of its characteristics.

Solutal advective flow induced in a horizontal shallow layer by the initial longitudinal steplike density distribution becomes unstable when the solutal P\'eclet number exceeds the critical value. The P\'eclet number uniquely determines the spatiotemporal characteristics of the secondary flow.

We report shock-wave-like structures that are strikingly different from previously observed fingering instabilities, which occur in a two-layer system of miscible fluids reacting by a second-order reaction A+B→S in a vertical Hele-Shaw cell. While the traditional analysis expects the occurrence of a diffusion-controlled convection, we show both experimentally and theoretically that the exothermic neutralization reaction can also trigger a wave with a perfectly planar front and nearly discontinuous change in density across the front. This wave propagates fast compared with the characteristic diffusion times and separates the motionless fluid and the area with anomalously intense convective mixing. We explain its mechanism and introduce a new dimensionless parameter, which allows to predict the appearance of such a pattern in other systems. Moreover, we show that our governing equations, taken in the inviscid limit, are formally analogous to well-known shallow-water equations and adiabatic gas flow equations. Based on this analogy, we define the critical velocity for the onset of the shock wave which is found to be in the perfect agreement with the experiments.

We report new chemoconvective pattern formation phenomena observed in a two-layer system of miscible fluids filling a vertical Hele-Shaw cell. We show both experimentally and theoretically that the concentration-dependent diffusion coupled with the frontal acid-base neutralization can give rise to formation of the local unstable zone low in density resulting in a perfectly regular cell-type convective pattern. The described effect gives an example of yet another powerful mechanism which allows the reaction-diffusion processes to govern the flow of reacting fluids under gravity condition., Comment: to be published in Physical Review E

The structure of the convective motion, which takes place after the vertical channel filled with two layers of miscible liquids is translated into a horizontal position, was studied experimentally. The characteristics and stability of the main flow were examined under various density differences and thicknesses of the channel. It is shown that the movement of the displacement front observed in all experiments can be described with a single curve by using certain nondimensional coordinates and time. It was found that an increase of density difference leads to instability of the main flow. This results in the appearance of a secondary flow in the form of helical convective rolls which form inside the mixing zone. Possible mechanisms responsible for the occurrence of such instability are discussed.