Effect Of Surfactant On Instability Of Front Waves In A Reaction-Diffusion-Convection System

Chemical reaction coupled with transport process can lead to intricate chemical spatiotemporal dynamics phenomena, such as chemical fronts, pulse, breath wave, and Turing pattern. Chemical reaction around the propagating front can generate various changes in physical properties of the system, such as density, surface tension and viscosity, through simultaneous conversion of material and energy. Density gradient induces buoyancy convection in the bulk solution, while surface tension gradient induces Marangoni convection at the free surface. Chlorite-trithionate reaction is a classic exothermic autocatalytic process in which hydrogen ions acts as an autocatalyst and chemical front propagates in a spatially extended system. Meanwhile trithionate is found rather stable in alkaline solution with appropriate hydroxide present as an inhibitor before front initialized. In this context, we study the instability of reaction-diffusion-convection front and the effect of surfactants in the chloritetrithionate reaction. In theoretical study, we simulate the front instability using coupled equations of multiple physical processes, revealing the underlying mechanism of the front instability.The influence of convection on horizontally propagating chemical front is studied experimentally using chlorite-trithionate reaction in a rectangular reactor. Oscillatory propagation of the front is observed for the first time under suitable conditions, including reactant concentration and the height of solution. The effect of surfactant on the front structure and stability is investigated in detail. Four kinds of surfactant are used in our study, i.e., sodium dodecyl sulfate(SDS), sulfobetaine 12(SB3-12), Triton X-100 and sodium dodecyl benzene sulfonate(SDBS). Convective rolls are observed in the existence of SDBS which concentration can influence the temperature across the front. Thermal effect is reduced with the addition of SDS, consequently the product lies beneath the reactant. Except SDS, other surfactants show a positive effect on the front velocity and maximum mixing length along the increase of their concentration. Moreover, the dependence of front velocity is also investigated on the reactant concentration and the height of solution.Using incompressible Navier-Stokes equation coupled with reaction-diffusion and heat equations, we numerically simulate the horizontally propagating of exothermic autocatalytic chemical fronts dominated by buoyancy convection and accompanied by Marangoni convection. Meanwhile, key parameters are estimated their effect on the front instability, including the density temperature coefficient of buoyancy convection and surface tension temperature coefficient of Marangoni convection.