LU12255EJ
- Waves of chemical reaction spreading through a heterogeneous media are found throughout biology, chemistry, and physics. Most theories to understand how these waves spread assume that individual particles can be neglected and their effect is smoothed over the media. Our paper shows that for some systems, this assumption fails and an unusual regime of wave propagation can occur which we call the “discrete regime.” Examples of familiar systems that might exhibit this behavior include clouds of combustible dust in air, forest fires, or flames propagating through a rocket propellant. In the discrete regime, even if the particles burn infinitely fast, the reactive wave (or flame) is still limited by the time it takes the heat released by one particle to spread to the neighboring particles. Therefore, the overall process becomes statistical, being influenced by the randomized position of the particles in three-dimensional space. In addition to theoretical and computer-based solutions, we experimentally observed this discrete regime by igniting flames in suspensions of iron particles in air, where the nitrogen had been replaced by xenon in order to decrease the heat conductivity of the gas. The slow propagation speed of the flames (3 to 5 cm/s) made them sensitive to being disrupted by gravity and necessitated conducting the experiments in a reduced-gravity (freefall) environment onboard an airplane flying along a parabolic trajectory.
