Surfactant-induced gradients in the three-dimensional Belousov-Zhabotinsky reaction

Abstract

Scroll waves are prominent patterns formed in three-dimensional excitable media, and they are frequently considered highly relevant for some types of cardiac arrhythmias. Experimentally, scroll wave dynamics is often studied by optical tomography in the Belousov-Zhabotinsky reaction, which produces CO(2) as an undesired product. Addition of small concentrations of a surfactant to the reaction medium is a popular method to suppress or retard CO(2) bubble formation. We show that in closed reactors even these low concentrations of surfactants are sufficient to generate vertical gradients of excitability which are due to gradients in CO(2) concentration. In reactors open to the atmosphere such gradients can be avoided. The gradients induce a twist on vertically oriented scroll waves, while a twist is absent in scroll waves in a gradient-free medium. The effects of the CO(2) gradients are reproduced by a numerical study, where we extend the Oregonator model to account for the production of CO(2) and for its advection against the direction of gravity. The numerical simulations confirm the role of solubilized CO(2) as the source of the vertical gradient of excitability in reactors closed to the atmosphere.

Inhibitory effect of oxygen on excitation waves in the Belousov–Zhabotinsky reaction with different excitability

“…In addition, a surfactant, 0.05 mM sodium dodecyl sulfate (SDS, Fluka), was added to the reaction medium to reduce the production of CO 2 bubbles [26]. It has been shown that such the low SDS concentration did not cause a significant effect on the excitability of the medium [33].…”

Helical deformation of the filament of a scroll wave

“…To reduce the production of CO 2 bubbles, the surfactant sodium dodecyl sulfate (SDS) was added at concentrations below its critical micelle concentration [18]. The initial concentrations were 50 mM malonic acid (Merck), 50 mM sodium bromate (Merck), 0.2 mM SDS (Fluka), and 0.5 mM ferroin.…”

“…Scroll waves were initiated by the partition method [6,7,18,22]. The reaction volume was divided into two compartments with a PET sheet and a cylindrical wave was initiated by immersion of a silver wire (diameter 0.5 mm).…”

“…For a given orientation of the gradient the drift direction depends on the sense of rotation of the scroll wave. Experimental realizations of twisted scroll waves were studied using gradients of temperature [16], illumination intensity [17], concentration of oxygen [6], surfactant [18], and electric current [19,20].…”

“…Thus, in experiments these two may not be easily distinguishable, although they are due to different mechanisms. The situation becomes even less unique when studying a meandering scroll wave in a medium with an excitability gradient [18].…”