Dynamic Instability And Control Of Spatiotemporal Patterns In The Chlorite-trithionate Reaction-diffusion-convection System

Chemical reaction can induce the flow of fluids,and hydrodynamic flow plays an active role on the chemical process simultaneously.The reaction-diffusion-convection(RDC)system coupled of the above two processes can generate various and abundant spatiotemporal self-organizing structures,which are ubiquitous in nature.Using chlorite-trithionate reaction as a carrier,we systematically analyzed the chemo-hydrodynamic front instabilities,such as fingering front,oscillatory propagation,and interfacial acceleration in the RDC system.The study methods for the RDC system ranged from experimental design,mechanical explanation to numerical simulation.We first studied the kinetics of the chlorite-trithionate reaction followed by the spatiotemporal dynamics in the reaction-diffusion(RD)system,which showed the advantages of the reaction for chemo-hydrodynamic study.Coupling with buoyancy convection and Marangoni effect,the RDC system presented different spatiotemporal patterns.The heat and solutal effects from the reaction could both service as controlling parameters in tuning the instabilities,which made us comprehensively and systematically understand the essence of the RDC behaviors.By studying the kinetics of the chlorite-trithionate reaction in a p H buffer,the reaction mechanism was proposed.The apparent reaction rate and the reaction orders against different reactants were also determined using the initial rate method.We presented a reaction model containing the main steps of the reaction,which laid the foundation for subsequent research.Advantages of chlorite-trithionate as a model reaction were as following.First,the fast reaction rate made the reaction possible to form RD/RDC front in proper time scale,which was convenient for experimental observation.Meanwhile,chemical stabilities of the two reactants,especially trithionate,made the reaction isolated from the formation of other sulfur-containing reductant.The single reaction of the hydrogen autocatalysis guaranteed the stability and controllability of the system.Based on these points,one-dimensional front study was carried out in the RD medium.The velocities under different combination of reactant concentrations were determined,which verified the rationality of our model.In the two-dimensional study,cellular front was observed through the diffusion control of the proton autocatalyst.The concentration range for the instability was also obtained by varying the initial concentrations of reactants.The convection was then introduced to the reaction system and typical Rayleigh-Taylor instability induced fingering was observed when the front propagated downward in the gravitational field.We determined the density change during the chlorite-trithionate reaction and found that the density of product was heavier than that of reactant at the same temperature.Consequently,the density difference caused the Rayleigh-Taylor instability.Using the thickness of solution layer as controlling parameter,we failed to observe the instability when the thickness was smaller than 0.3 mm.However,the front velocity,mixing length,and wavelength all increased along with the widening of solution layer.Furthermore,the reaction heat generated by the thick layer reversed the density difference between reaction and product,making fingering instability during upward propagation.The initial concentration was also used to tuning the density change and motion of the front because different end products were produced and made different situation for the propagation.Additionally,the pH indicator in a high concentration may become key parameter in controlling the front instability.We numerically simulated the evolution of the front using finite element method.The Brinkman-Stocks equation was applied to construct a RDC model by coupling to multiple reaction steps.The simulated results agreed well with the experimental results.Horizontally propagation of the chlorite-trithionate RDC front was also studied in the Hele-Shaw cell.When the interfaces of the solution were all kept closed,lateral instability was observed.Due to the Rayleigh-Taylor instability,the product sinked under the reactant,making a bent waveform.The density reverse was caused by the increase of solution layer which strengthened the reaction heat effect.Desynchrony of heat and solutal effect of the density resulted in an oscillatory propagation of the front.With the upper surface open to the air,the Marangoni convection,induced by gaseous chlorine dioxide which was produced from chlorite disproportionation after the proton autocatalysis,greatly strengthened the superficial convection.The using of indicators Congo red and bromophenol blue could eliminate and reserve the Marangoni effect,respectively.Combined effect of buoyancy and Marangoni convection was also investigated by coupling reaction heat induced density change.Numerical study was carried out to solve the model proposed by coupling Navier-Stokes equation with reaction-diffusion and heat equations.Consistent results between experiment and simulation were obtained under multiple convection.Effect of inert surfactant on the front instabilities was studied in the reactor with an open surface to the air.The triton x-100 was found to strengthen the propagation of upper layer of the front,leading to faster velocity and larger mixing length.However,in the presence sodium dodecyl sulfate,the front kept down-titled waveform because the propagation in the upper layer was suppressed.Through the determination of density,temperature,surface tension and surface diffusion,we deduced that temperature gradient induced by the reaction heat was the main cause for the enhancement of convection in the system.Extra Marangoni effect generated from the temperature gradient at the interface where surfactant was concentrated.Moreover,liquid film comprised of nonionic surfactant significantly increased the surface diffusion of autocalayst.Based on the former numerical model,we qualitatively simulated the experimental processes through the introduction of surfactant mass transfer process and temperature response coefficient of surfactants.In conclusion,based on the chlorite-trithionate reaction,we design and control the spatiotemporal dynamics and self-organizing structures of the RDC system under the effect of different hydrodynamic instabilities.The incorporation of convection to the RD system reflects the self-organizing phenomena in living organism and inanimate system more realistically.This study may be beneficial and heuristic to the studies concerning RDC process such as process engineering and drug delivery.