An experiment showing the formation of a flame structure. (a) The experimental cell is shaken for 5 s at an acceleration of 40.5 m/s2, a frequency of 40 Hz, and an amplitude of 0.64 mm. The two yellow lines trace the surface of the granular layer and the two-layer interface. The two light blue reference lines at 5 s indicate these heights at t = 0 s. (b) A close-up of the two-layer interface at t = 1.6 s indicated by a rectangle in (a). The measured wavelength of the instability is λ = 3 ± 1 mm (average and standard deviation for three wavelengths). (c) Same as (b) but at t = 5 s with λ = 21 ± 6 mm (average of three wavelengths). (d) Time evolution of the two-layer interface topography with time. The stars indicate the same timing as shown in (a). Note that the amplitude and the wavelength increase with time.