PHYSICAL REVIEW E 86, 016320 (2012) Upscaling energy concentration in multifrequency single-bubble sonoluminescence with strongly degassed sulfuric acid Dami´ an Dellavale, Ludmila Rechiman, Juan Manuel Rossell´ o, and Fabi´ an Bonetto Instituto Balseiro-CONICET, Centro At´ omico Bariloche, R´ ıo Negro, R8402AGP, Argentina (Received 6 April 2012; revised manuscript received 3 June 2012; published 20 July 2012) Single-bubble sonoluminescence (SBSL) was explored under a variety of multifrequency excitations. In particular, biharmonic excitation was used to produce SBSL for unprecedented low dissolved noble gas concentrations in a sulfuric acid solution. Reducing the amount of dissolved noble gas makes it possible to reach higher acoustic pressures on the SL bubble, which otherwise are not attainable because of the Bjerknes instability. By using biharmonic excitation, we were able to experimentally trap and to spatially stabilize SL bubbles for xenon pressure overhead as low as 1 mbar. As a result, we have access to regions in phase space where the plasma temperatures are higher than the ones reached before for bubbles driven at ≈30 kHz. DOI: 10.1103/PhysRevE.86.016320 PACS number(s): 78.60.Mq I. INTRODUCTION It has long been known that, in order to increase energy concentration in single-bubble sonoluminescence (SBSL), avoiding the abrupt extinction of the SL bubble (Rayleigh- Taylor and parametric shape instabilities), increasing the acoustic pressure on the bubble (P LF,b ) while keeping the SL bubble at the resonator center (positional stability), and decreasing the amount of dissolved noble gases (minimum ambient radius R 0 ) are needed [1–3]. However, to access the regions of parameter space with minimum R 0 and maximum P LF,b has proven to be a challenge to the experimentalist. Flannigan and Suslick [4] used a sulfuric acid (SA) aqueous solution that allowed a significant increase in luminescence. One of the limiting mechanisms of energy concentration upscale in SA is the positional instability of the SL bubble. We showed that, due to the mean primary Bjerknes force, the bubble reaches a position where the acoustic pressure is always the same irrespective of the pressure in the center of the resonator [5]. A further noteworthy complication of SBSL in SA originates from the movement of the SL bubble in quasiperiodic orbits (spatial instability) [6,7]. Toegel et al. [7] showed that the history force is responsible for the orbits. Urteaga and Bonetto [8] suppressed the orbits using a biharmonic (two frequencies) excitation. In this paper, we have used an aqueous solution that is 85% sulfuric acid by weight (SA85) since its high viscosity prevents Rayleigh-Taylor shape instability. We also used a very low concentration of noble gas (xenon pressure overhead of 1 mbar) to investigate the regions of parameter space R 0 − P LF,b where the high maximum bubble temperatures (T max ) are expected (minimum R 0 , maximumP LF,b )[1–3]. Using a particular biharmonic driving, we achieved unprecedented low R 0 and high P LF,b upscaling the energy concentration at the bubble collapse. Under these conditions, the computed plasma temperatures are higher than those achieved for bubbles driven with single- frequency (≈30 kHz). II. APPARATUS We used a spherical flask made of quartz (60 mm in outer diameter and approximately 1 mm in thickness). In all cases presented here the liquid was SA85. The multifrequency driving was applied through four PZT drivers glued on the flask. Two opposed PZT drivers were used for the low- frequency driving (V LF at f 0 ), and the other two PZTs were used for the high-frequency driving (V HF ) similar to the arrangement in Ref. [8]. In all the experiments described below, we used, as the fundamental frequency, the one corresponding to the resonator first mode (f 0 ≈ 29.2 kHz). We developed a tailored system for concurrent signal synthesis and measurement based on field programmable gate array (FPGA) technology [9,10]. The high-frequency signal (V HF ) was amplified using a custom-built low distortion high voltage amplifier (B w = 400 kHz at C L = 2 nF) to excite the high- frequency drivers. A small piezoelectric ceramic microphone was glued to the resonator wall in order to obtain a signal proportional to the applied acoustic pressure at the center of the resonator (P LF ). We also detected the SL pulse using an Oriel 77340 phototube. The use of a timer (Stanford Research Systems SR620) allowed us to measure the time of collapse (t c ), defined as the time interval between the acoustic pressure zero crossing with negative slope and the SL pulse arrival. The time precision of the measurements was only limited by the microphone signal jitter to about 100 ns. The timer was operated at its maximum rate (≈1500 samples/s). The SL intensity, stability, and position of the SL bubble within the resonator were provided by a CCD camera (Hitachi KPF120). To measure the bubble radius temporal evolution [R(t )], we used conventional Mie light scattering techniques [11]. In order to determine all the quantities of interest, the Mie scattering data were matched to a simulation obtained with a state-of-the-art model described in Ref. [12] (and references therein). III. SPATIAL INSTABILITY In order to prevent the errors of the Mie scattering data caused by the bubble pseudo-orbits, we produced spatially stable SL bubbles using one of the two mechanisms: (1) We produced almost fixed SL bubbles at the resonator center by using single-frequency driving (f 0 ) and low P LF [zone (A) in the left photograph of Fig. 3). (2) Alternatively, we remove the spatial instability using harmonic excitation (Nf 0 ), ranging from N = 3(≈88 kHz) to N = 7(≈205 kHz) besides the 016320-1 1539-3755/2012/86(1)/016320(6) ©2012 American Physical Society