Interaction of Pure Marangoni Convection with a Propagating Reactive Interface under Microgravity.

A reactive interface in the form of an autocatalytic reaction front propagating in a bulk phase can generate a dynamic contact line upon reaching the free surface when a surface tension gradient builds up due to the change in chemical composition. Experiments in microgravity evidence the existence of a self-organized autonomous and localized coupling of a pure Marangoni flow along the surface with the reaction in the bulk. This dynamics results from the advancement of the contact line at the surface that acts as a moving source of the reaction, leading to the reorientation of the front propagation. Microgravity conditions allow one to isolate the transition regime during which the surface propagation is enhanced, whereas diffusion remains the main mode of transport in the bulk with negligible convective mixing, a regime typically concealed on Earth because of buoyancy-driven convection.

Buoyancy-driven convection may switch between reactive states in three-dimensional chemical waves.

Traveling waves in an extended reactor, whose width cannot be neglected, represent a three-dimensional (3D) reaction-diffusion-convection system. We investigate the effects of buoyancy-driven convection in such a setting. The 3D waves traveled through horizontal layers of the iodate-arsenous acid (IAA) reaction solution containing excess of arsenous acid. The depth of the reaction solution was the examined parameter. An increase in the intensity of buoyancy-driven flow caused an increase of the traveling wave velocities. Convection distorted the front of the chemical waves. For layers deeper than h>13 mm, heat release became smaller than heat production causing the emergence of Rayleigh-Bénard convection cells. At the interface, a dependency of wave shape on solution depth was observed. For h<7 mm, the waves adopted a stable V-like shape, while for h>13 mm a parabolic shape dominated. For 7

Modeling of glycolytic wave propagation in an open spatial reactor with inhomogeneous substrate influx.

The spatio-temporal dynamics of traveling waves in glycolysis as it occurs in yeast extract have been studied, both theoretically and experimentally. We describe this phenomenon with the distributed Selkov model that accounts for the reactions of phosphofructokinase, which is a key enzyme of the glycolytic reaction cascade. To describe the experimentally observed phase waves in an open spatial reactor we introduce a non-homogeneous flux of substrate in the model. The experimental observation that waves can change their direction of propagation during the experiment is considered in the model. The mechanism for such a change in wave direction is discussed.

Wavy fronts and speed bifurcation in excitable systems with cross diffusion.

A bifurcation of excitation fronts induced by cross diffusion in two-component bistable reaction-diffusion systems of activator-inhibitor type is discovered. This bifurcation is similar to the nonequilibrium Ising-Bloch bifurcation. A different type of fronts, whose spatial profiles are characterized by oscillating tails, are associated with this bifurcation. These fronts are described using exact analytical solutions of piecewise linear reaction-diffusion equations.

Flow-field development during finger splitting at an exothermic chemical reaction front.

Fingertip splitting may be observed at chemical reaction fronts subject to buoyancy-induced Rayleigh-Taylor fingering, as investigated in ascending fronts of the iodate-arsenous acid reaction in vertical Hele-Shaw cells. We study the properties of the flow-field evolution during a tip-splitting event both experimentally and theoretically. Experimental particle-image velocimetry techniques show that the flow field associated to a finger displays a quadrupole of vortices. The evolution of the flow field and the reorganization of the vortices after a tip-splitting event are followed experimentally in detail. Numerical integration of a model reaction-diffusion-convection system for an exothermic reaction taking into account possible heat losses through the walls of the reactor shows that the nonlinear properties of the flow field are different whether the walls are insulating or conducting. In insulating systems, the flow field inside one finger features only one pair of vortices. A quadrupole of flow vortices arranged around a saddle-node structure similar to the one observed experimentally is obtained in the presence of heat losses suggesting that heat effects, even if of very small amplitude, are important in understanding the nonlinear properties of fingering of exothermic chemical fronts.