Chemical Reaction Pattern Shines Like the Sun

David Ehrenstein

Video

Chemical Reaction Pattern Shines Like the Sun

February 13, 2026• Physics 19, 21

A chemical reaction involving a flowing fluid leads to a striking visual pattern—a result that could benefit research on other propagating fronts, such as flames.

S. N. Maharana et al. [1]

In this simulation, the colors represent the concentration of X in the reaction 2X + Y $\to$ 3X, with red indicating the largest value and blue indicating zero. Y is fed in continuously from the center, at first pushing the reaction front outward, but eventually the front stops moving when the outward flow matches the rate at which Y is consumed. The “sun-ray” pattern then forms as a result of the assumed difference between the X and Y diffusion coefficients.

For a phenomenon like a wildfire burning through a forest or a disease moving through a population, the resulting patterns can sometimes be modeled using a so-called reaction–diffusion system—an experiment where a chemical reaction front moves through a region full of reactants. Now Anne De Wit of the Université Libre de Bruxelles and her colleagues have demonstrated that new patterns can be revealed when the reactants flow against the direction of the front’s propagation, causing it to freeze in place [1]. Their “sun-ray” pattern is the first one discovered this way, but the technique could generate other patterns that might replicate behavior in forest fires or epidemics.

S. N. Maharana et al. [1]

Experimental implementation of the simulation above using the chlorite–tetrathionate reaction [3].

Two years ago, De Wit and her colleagues brought a propagating reaction front to a standstill by slowly and continually injecting a reactant into the center of a disk-shaped chamber filled with the other reactant, against the front’s inward propagation [2]. The stopping occurred when the outward flow matched the rate at which the inner reactant was consumed. De Wit says that a stationary front allows more control and thus more careful study of patterns than a propagating front.

As a demonstration of this control, the researchers have now used reactants with different diffusion rates in the same outward-flow setup. In this case, the stationary front developed ripples—an effect previously seen in propagating fronts. The researchers also observed radial “rays”—narrow regions of higher concentration of one of the reactants. They showed in simulations and experiments that properties of the front can be precisely controlled by varying the flow rate.

Figure caption — **Stabilizing an instability.** At the front of the reaction 2X + Y $\to$ 3X, small bumps can grow when Y diffuses faster than X. The reason is that the reaction rate along the front depends on a bump’s curvature, which can either concentrate Y (right-protruding bumps) or concentrate X (left-protruding bumps). When this concentration effect is combined with a faster diffusing Y, the reaction rate in right-protruding bumps will exceed that of left-protruding bumps, which means that the bump sizes will increase as the front propagates to the right.

Other patterns may be possible. De Wit plans to try turning other knobs, such as introducing differences between the reactants’ viscosities or buoyancies. “As a chemist, I am fascinated to see how the self-organizing power of chemistry can bloom” when reactions involve fluid flow, De Wit says. “That’s the central goal of the research on chemohydrodynamics that I am trying to develop.”

–David Ehrenstein

David Ehrenstein is a Senior Editor for Physics Magazine.

References

S. N. Maharana et al., “Radially locked sun-ray patterns in reaction-diffusion-advection systems,” Phys. Rev. Lett. 136, 068001 (2026).
L. Negrojević et al., “Frozen autocatalytic fronts in a radial flow,” Phys. Rev. Res. 6, L042044 (2024).
D. Horváth and Á. Tóth, “Diffusion-driven front instabilities in the chlorite–tetrathionate reaction,” J. Chem. Phys. 108 (1998).