Differential diffusion effects on buoyancy-driven instabilities of acid–base fronts: the case of a color indicator

Abstract

Buoyancy-driven hydrodynamic instabilities of acid-base fronts are studied both experimentally and theoretically in the case where an aqueous solution of a strong acid is put above a denser aqueous solution of a color indicator in the gravity field. The neutralization reaction between the acid and the color indicator as well as their differential diffusion modifies the initially stable density profile in the system and can trigger convective motions both above and below the initial contact line. The type of patterns observed as well as their wavelength and the speed of the reaction front are shown to depend on the value of the initial concentrations of the acid and of the color indicator and on their ratio. A reaction-diffusion model based on charge balances and ion pair mobility explains how the instability scenarios change when the concentration of the reactants are varied.

Mixed-mode instability of a miscible interface due to coupling between Rayleigh-Taylor and double-diffusive convective modes

“…As color indicator or dyes can perturb the dynamics, 25,26 visualization is made by a Schlieren technique 27 tracking changes in the index of refraction induced by the dynamics. In effect, the experimental images (Fig.…”

Asymmetric Rayleigh-Taylor and double-diffusive fingers in reactive systems

“…[16][17][18] and references therein) and combustion problems. 12,19 In parallel, the coupling between convective motions and a simple A + B → C reaction has been addressed both experimentally [20][21][22][23][24][25][26][27][28][29] and theoretically. 24,27,[29][30][31][32] In the specific a) L. Lemaigre and M. A. Budroni contributed equally to this work.…”

Cross-diffusion-induced convective patterns in microemulsion systems

“…31 In these instability scenarios, when the density barrier below the interface increases, the characteristic time scale for the onset of the hydrodynamic instability increases and, conversely, convective motions are less intense, as the density extrema that are responsible for the convective motions progressively decrease along the global density profile. 33 In our experimental investigations we found a decreasing trend for all the hydrodynamic parameters upon increasing the initial concentration of the salt in the bottom layer. At present it is difficult to give a full explanation for these trends, because the exact dependence of the cross-diffusion coefficients upon [NaBrO 3 ] must be known.…”

“…The parameters which characterize the instability dynamics are related to the ratio between the difference in density across the interface (density maximum and minimum above and below the interface, respectively) and the initial density difference between the top and bottom layer. 33 In our model, this ratio remains constant with increasing DC 0 , the latter being a proportional factor for both terms. However, Fig.…”