Single Bubble Sonoluminescence and Bubble Surface Stability in Surfactant Solutions

The radial dynamics of a single sonoluminescing bubble has been investigated in surfactant solutions. Experimental results show that an increase in the surfactant concentration leads to a decline in the oscillation amplitude and hence light emission intensity. Numerical simulations support this result, showing that under the driving pressures required to achieve single bubble sonoluminescence (SBSL), the surface properties, namely the surface elasticity and dilatational viscosity, contribute to the damping of the radial amplitude in the bubble oscillation. In most cases this stabilises the bubble surface, but leads to a decreased light intensity due to smaller oscillation amplitude. The application of a stronger driving pressure in an attempt to produce equivalent light emission to a surfactant-free bubble, leads to a decrease in the surface stability, making it practically very difficult for a bubble to achieve high SBSL intensities in concentrated surfactant solutions. Although the bubble oscillates at a smaller amplitude, the instability mechanism for a surfactantcoated bubble at higher ambient radii and surfactant concentrations, is more likely to be of the Rayleigh-Taylor type than that of a clean bubble at the same given acoustic parameters. This can lead to bubble disintegration before correcting mechanisms can bring the bubble back into the stable SL regime. Introduction A single bubble levitated in a standing wave can, under specific conditions, experience nonlinear pulsations that result in light emission. This phenomenon is called single bubble sonoluminescence (SBSL) [6]. A bubble smaller than the resonance size injected into a standing wave field, will be drawn towards the pressure antinode due to the Primary Bjerknes force [3, 7]. The light intensity emitted by this pulsating bubble, is dependent on various factors that include the amount and type of dissolved gases in the liquid [20], the frequency of the applied ultrasound [2], the applied sound pressure amplitude, hydrostatic pressure and addition of particular solutes [1, 14, 16, 19]. Surface active solutes, i.e., surfactants, have been shown experimentally to influence the behaviour of SBSL. Ashokkumar et al. [1] showed that micromolar concentrations of non-volatile surfactants such as sodium dodecyl sulphate (SDS), dodecyl trimethyl ammonium chloride (DTAC) and decyl ammonium propane sulfonate (DAPS) did not significantly affect the dynamics or SL of a single bubble. Numerical simulations performed by Yasui [18] explained that the effect of the surfactant was to inhibit the condensation of water vapour at the bubble wall during bubble collapse which lowers the achievable temperature inside the bubble. The shape stability of a bubble is another important consideration. Instabilities arise from perturbations of the surface during oscillation that disrupt the spherical shape of the bubble such that the curvature of the liquid becomes non-uniform and form a local surface tension pressure associated with each point of the surface [8]. Under stable conditions, these perturbations are dampened and the bubble returns to its equilibrium condition (spherical). However, sometimes, dramatic overshoot can occur which can propagate over a large number of cycles (parametric instability), leading to experimentally observed phenomena such as shape mode oscillations [15]. In some cases, dramatic oscillations occur at the point of a strong bubble collapse and persist only for a single cycle (Rayleigh-Taylor instability) which may cause a bubble to move chaotically (dancing motion) and to pinch-off daughter bubbles [5] or to disintegrate completely, as the bubble usually does not have enough time to correct the strong perturbation to its surface. In surfactant rich environments, the bubble surface will have viscoelastic properties that may dampen or enhance the shape stability. The effect of higher concentrations of surfactant on SBSL, has recently been studied by Leong et al. [10] experimentally and theoretically. This conference presentation will report on the findings of this work. Materials and Methods Figure 1. Set-up for measuring the SBSL intensity and radial dynamics from Leong et al. [10] Copyright (2014) by the American Physical Society. Further details of the experimental setup and methods can be found in Leong et al. [10] The surfactants used in the SBSL experiments were of the purest grades available: Sodium dodecyl sulphate (SDS) (VWR international, purity>99%) and dodecyl trimethyl ammonium chloride (DTAC) (TCI Japan, purity>99%). Sodium chloride (NaCl) was supplied by Merck Germany (purity>99.5%). The same apparatus set-up as detailed for single-bubble rectified diffusion experiments by Leong et al. [9] was used with minor adjustments in these experiments (figure 1). A vacuum pump was used to partially degas the solution. To determine the bubble’s radial dynamics, light emitted from a low power laser diode (633 nm) was directed at the bubble and its scattered intensity was measured using a photomultiplier tube (PMT) (Hamamatsu E849-35 amplified by Canberra H.V. Supply Model 3002). The same PMT was also used to measure the sonoluminescence intensity. The PMT signal was relayed to an oscilloscope (LeCroy WaveSurfer 452) and an average over 50 sweeps was taken. A driving pressure between 1.1 to 1.3 bar and frequency of between 22.23 and 22.31 kHz were used. The maximum bubble radii were determined using images processed in ImageJ. The minimum bubble radii could not be determined from the images taken. Instead, an approximate Rmax/Rmin ratio was determined from the data obtained by the oscilloscope for the reflected laser light, which was plotted in Matlab for further analysis. The estimated Rmin in this case, was in the order of 5 μm radius. Equations The equation of motion used to calculate the radius of a bubble in an acoustic field is a modified Keller Equation adapted from Yasui [17]: