Temperature Inhomogeneity during Multibubble Sonoluminescence

When a liquid is subjected to high-intensity ultrasound, bubbles are formed, grow, and implosively collapse. This phenomenon of acoustic cavitation generates both chemical reactions (i.e., sonochemistry) and the emission of light (i.e., sonoluminescence, SL). It is generally agreed that both sonochemistry and sonoluminescence result from the intense compressional heating of gas and vapor inside the collapsing bubbles, and the extraordinary temperatures and pressures thus created. The emission of light can occur either from a cloud of cavitating bubbles (i.e., multibubble sonoluminescence, MBSL), or in a carefully controlled standing wave acoustic field from a single isolated bubble (i.e., single-bubble sonoluminescence, SBSL). MBSL is more closely related to sonochemistry, and quantification of the conditions generated during MBSL can lead to a better understanding of sonochemistry. Measurement of atomic and molecular emission from volatile species during MBSL revealed effective temperatures of thousands of Kelvins created during bubble collapse. Little is known, however, about the origin of emission derived from nonvolatile species during MBSL. This emission is directly relevant to the observed sonochemistry of dissolved reactants. Extensive emission bands and lines have been observed both from aqueous and non-aqueous liquids during MBSL, and can be used as spectroscopic thermometers to quantify the conditions generated inside the collapsing bubbles. For example, the Swan bands of C2 , [8,14] excited-state metal atoms (e.g., Fe, Cr, Mo), and even excited state Ar emission have been used to measure the intracavity temperatures. Other nonspectroscopic methods have also been used to measure the temperatures of cavitating bubbles. No prior study, however, has reported simultaneous measurement of temperature from two or more independent emitting species, which would permit one to probe the homogeneity of the temperature profile generated in bubble clouds from spatial variance during acoustic cavitation. By examining the MBSL from aqueous H3PO4 solutions, we have observed ultrabright sonoluminescence, found strong molecular emissions from both OHC and POC radicals, and have succeeded in using both simultaneously as spectroscopic thermometers. There is a dramatic temperature inhomogeneity that is dependent on the location within the bubble cloud and is consistent with two distinct kinds of cavitating bubbles: those that collapse symmetrically and those that do not. H3PO4 is a strongly hydrogen-bonded liquid; it has a relatively high viscosity and low vapor pressure (ca. 2.4 Torr for 85%H3PO4). Interestingly, the vapor of H3PO4 consists of water molecules alone; there are no acid molecules present in the vapor over most concentrated H3PO4 samples, even at high temperatures. Thus, in the gas phase of H3PO4 , the only volatile component inside the bubbles is water vapor; the phosphoric acid molecules can be considered as nonvolatile species during MBSL. Ultrabright sonoluminescence from 85% H3PO4 saturated with noble gases can be observed by naked eye, even in a well-lit room, as shown in Figure 1.