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field distribution22. The acoustic signals that cavitating bubbles emit (Fig. 1c) were measured using a calibrated needle hydrophone. Inertial cavitation was delineated from stable cavitation by quantifying the broadband noise, over a MHz frequency range in which harmonic activity is not distinguishable from the background noise23. This was carried out by measuring the high frequency broadband energy24 (equation 1), parametrised as πΈπππ£. 1ππ πΈ = β β«2π(π)2ππ (1) πππ£ π π‘=1 π1 π where π (π) are the spectral magnitudes measured from the frequency domain cavitation spectra π (Fig. 1c) and π and π are 1.5 MHz and 2.5 MHz respectively. The inertial cavitation threshold was 12 determined by measuring πΈπππ£ as a function of the nominal input power of the vessel; the measurement protocol is described in the Supplementary Information 1. As shown in Fig. 1d, the inertial cavitation threshold is characterised by a systematic rise in πΈπππ£23. This occurs above a pre- amp voltage of ~60 mVRMS, which corresponds to a nominal input electrical power of 5 Watts (corresponding to a vessel power density of around 0.3 W/L). To study graphene exfoliation arising from the physiochemical effects of acoustic cavitation, samples were produced by sonicating graphite in 28 ml low density polyethylene vials positioned in a region where inertial cavitation activity is intense and localised (Fig S1.2a). Preliminary experiments found that graphene is first produced only after the onset of the inertial cavitation (Fig. 1d), which demonstrates that the physiochemical effects of inertial cavitation drive graphene exfoliation during ultrasonication. At high pre-amp voltages (high acoustic powers) πΈπππ£ saturates due to cavitation shielding25, where a significant volume fraction of cavitating bubbles dynamically scatter and absorb the acoustic field. This considerably affected the graphene exfoliation rate such that a sharp reduction in the graphene yield occurred when πΈπππ£ saturates (Fig. 1d). The highly non- linear nature of inertial cavitation combined with the significant perturbation of the graphene exfoliation rate at high acoustic powers (Fig. 1d) suggests that measurement and control of inertial cavitation is essential when developing sonication methodologies. 6PDF Image | Cavitation Liquid Phase Exfoliation of Graphene
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