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The sonoluminescence generated in water with pulsed 515 kHz ultrasound has been studied in the presence of different chain length (C1−C5) aliphatic alcohols and the surfactants sodium dodecyl sulfate (SDS), dodecyltrimethylammonium chloride (DTAC), and N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DAPS). The ultrasound pulse widths used ranged from 1 to 10 ms, with duty cycles (on/off ratios) of 1:3 to 1:9. It was found that the sonoluminescence from the initial pulses was very low but increased in intensity and reached a maximum after 20−50 pulses, for all systems studied, depending on the pulse width and duty cycle. In the presence of alcohol the maximum signal decreased with increasing alcohol concentration, and the signal decline was more pronounced with increasing chain length of the alcohol. A good correlation was found to exist between the decline in the sonoluminescence signal and the Gibbs surface excess of the alcohol at the air/water interface. In the presence of SDS (an anionic surfactant) and DTAC (a cationic surfactant), quite different behavior was observed. At low concentrations of these two surfactants the maximum signal was significantly enhanced over that obtained in pure water, reaching a maximum at about 1 mM of surfactant. At higher concentrations the signal decreased again reaching a limiting value similar to that obtained in pure water. The sonoluminescence signal in DAPS (a zwitterionic surfactant) solutions remained much the same as in pure water. On the addition of 0.1 M NaCl to the three different types of surfactant solutions, the intensities of the emission signals obtained were essentially the same as in pure water. Possible mechanisms responsible for the different behavior in the sonoluminescence signal in the presence of the alcohols and surfactants are discussed.

The understanding of static interactions in colloidal suspensions is well established, whereas dynamic interactions more relevant to biological and other suspended soft-matter systems are less well understood. We present the direct force measurement and quantitative theoretical description for dynamic forces for liquid droplets in another immiscible fluid. Analysis of this system demonstrates the strong link between interfacial deformation, static surface forces, and hydrodynamic drainage, which govern dynamic droplet-droplet interactions over the length scale of nanometers and over the time scales of Brownian collisions. The results and analysis have direct bearing on the control and manipulation of suspended droplets in soft-matter systems ranging from the emulsions in shampoo to cellular interactions.

Acoustic bubble-size distributions have been determined using a pulsed ultrasound method at different ultrasound powers and frequencies. It was observed that the mean bubble size increased with increasing acoustic power and decreased with increasing ultrasound frequency. It was also recognized that the mean size of bubbles emitting sonoluminescence was greater than those producing sonochemiluminescence indicating that the two processes take place in different populations of cavitation bubbles in the system.

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