Figure 2
Displacement bursts and ultrasound emissions in indentation of suspended Mica-films. (a) Schematic representation of indentation. Thin Mica sheet is suspended over 8 piezoelectric transducers. The spheres correspond to locations where acoustic phonons are emitted as determined by a source location algorithm. The mica film covers all sensors and slightly exceeds the source-receiver distance. (b) Two loading-unloading paths with clear bursts events accompanied by emission of ultrasound waves. Inset shows an array of recorded emissions during the loading stage. (c) Evolution of the average of all nodes’ degree <k> for waveforms shown in (b) showing two main stages of relaxation. (d) High frequency components of an excited signal in our indentation test are mapped onto fast-weakening phase in <k(t)> onset of the fast-slip coincides with the broadening of the power spectrum. An overlapped (80%) 2,048-point fast Fourier transform is used to calculate the power spectral density. In (e) we show 5 different acoustic events transformed to <k(t)>, where <k(t)> represents time-evolution of the mean dynamic strain field over the spatially distributed sites. After initial rising phase (Phase I), a fast relaxation phase as the sharp drop of strain (phase II) is followed. Further relaxation occurs on longer timescales. See Fig. S1 for full waveforms of the shown k-profiles. (f) Accumulated k-chain patterns in ≈1.25 µs time-interval in transition from phase I to phase II for the plot shown in (c). The plot is the snapshot in vicinity of the peak of <k>; blue and red arrows show the trend of evolution or polarization of the chain in terms of the positive (growth-↑) or negative (folding-↓) rates respectively. Also see Supplementary Movie 1 as a 3d visualization of accumulated k-chains.
