Frequency ( f ) dependence of amplitude ( δz ) growth in the range of f = 10-5,000 Hz at fixed acceleration of 5.1 ± 0.3 m/s2
. The thin, medium, and thick lines correspond to the percolation, transition, and flame regimes, which are classified using the relative amplitude δ z′ at t = 9.9 s. The growth of the instability is fastest and attains the largest amplitude at an intermediate frequency of 150 Hz. A red arrow indicates a peak amplitude for the case in which a flame structure forms. Diameters of red and white particles which comprise the lower and upper layers, respectively, are indicated for comparison. Here, the amplitude data are smoothed (five-point running average) for the ease of visualization.