Spatiotemporal Dynamics Of Nonlinear Chemical Systems Coupling With Stimulus-responsive Gels

Stimuli-responsive soft materials coupling with nonlinear chemical reaction are thesuitable mediums for studying the spatiotemporal dynamics of self-organizing behaviors.The coupling system away from non-equilibrium conditions can show many nonlinearspatio-temporal dynamic phenomenons, such as chemical pulse waves, self-oscillation ofvolume changes and complex patterns. This paper mainly focuses on the experimentalphenomena, mechanism explanation and numerical simulation of the spatio-temporaldynamical behaviors in the ruthenium catalyzed Belousov-Zhabotinsky （BZ） gel system. Inexperiment, the catalyst （Ru（bpy）3） with double bond was graft copolymerized with thepolymer chains. Polymer network structure can effectively eliminate the convection effectduring the reaction-diffusion process. It’s feasible to get the stable spatio-temporaldynamics. We can get this functional gels coupled with BZ reaction and monitored thespatio-temporal dynamical behaviors at different experiment conditions.We first synthesized the BZ gel in one-dimensional capillary, with one end opened and theother sealed. Once the “dead-end” capillary filled with BZ gel touched with BZ solution.Wave initialized from the open end of the tube traveled a finite distance before dissipating.Remarkably, the propagation distance of successive waves was not constant but ratherexhibited complex patterns: multiple-length-scale propagation modes of chemical pulse wavesstarted with P¹and evolved orderly to P², P⁴, P⁸and P¹⁶with [NaBrO3]0increased, and finallypropagated into mixed waves. Our results demonstrated that the concentration of NaBrO3canbe used as control parameter to modulate the bifurcations of period-doublingmultiple-length-scale propagation modes. The spatiotemporal patterns were reproduced by areaction-diffusion model using the5-variable Oregonator model for the BZ kinetics. In thesimulate course, we modulated the period of chemical pulse waves by increasing A0（theconcentration of BrO3-）. We can see that there is also an evolution of period-doublingbifurcation phenomena with A0increased, which is in complete agreement with theexperimental results. The propagation modes evolved from simple to complex. Change otherexperimental parameters can also modulate the spatio-temporal dynamics. Our works aretrying to set up a useful model system not only for studying the nonlinear chemical dynamics,but also for understanding the multiple scale behaviors in nature. Such as the growth of plant,nutrients are diffusion-fed from the trunk and branches at multiple levels.On the other hand, the catalyst （Ru（bpy）3） grafted to the polymer molecular chains hasunique photosensitive characteristic. We further investigated the dynamic behavior ofhydrogels, in which Belousov-Zhabotinsky （BZ） reactions take place under the influence of irradiated light. BZ gels undergo a net displacement that is opposite to the propagation oftraveling waves, since these waves “push” the solvent away from the wave source. Amongregions of different oscillatory frequencies, the highest frequency determines the ultimatedirection of motion of the wave. So, we further studied changes in the frequency of theoscillating reaction upon irradiation of light with different intensities. It is found that thefrequency of the reaction first increases （photoinduction）, before it decreases monotonouslyand finally becomes arrested （photoinhibition）. Related photosensitive results are reproducedby a two-variable photosensitive Oregonator model. These photosensitive characteristics areused to design the experiment, in which differential irradiation of distinct segments of a BZgel is used to induce spontaneous motion either towards （phototropism） or away from（photophobism） brighter regions. These controlled behaviors are explained on the basis of theintensity-dependent frequency of the reaction, and related changes in the net displacement ofthe gel body （movement towards regions of higher frequency）. The experiments andsimulations demonstrate that photosensitive gels can be experimentally controlled bymanipulating these two photoeffects. This approach may be helpful in designing intelligentsoft robots capable of executing bionic functions.