ARTICLE pubs.acs.org/JPCC
The Role of Surfactant Headgroup, Chain Length, and Cavitation Microstreaming on the Growth of Bubbles by Rectified Diffusion Thomas Leong,† James Collis,‡ Richard Manasseh,§ Andrew Ooi,‡ Anthony Novell,|| Ayache Bouakaz,|| Muthupandian Ashokkumar,*,^ and Sandra Kentish*,† †
Department of Chemical and Biomolecular Engineering, The University of Melbourne, VIC 3010, Australia Department of Mechanical Engineering, The University of Melbourne, VIC 3010, Australia § Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, VIC 3122, Australia UMR Inserm U930, Universite Franc-ois Rabelais Tours, Tours 37044, France ^ School of Chemistry, The University of Melbourne, VIC 3010, Australia
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ABSTRACT: The role of a surfactant on the rate of bubble growth via rectified diffusion during acoustic cavitation has been investigated. The charge of the headgroup of ionic surfactants did not affect the bubble growth rate at low surface loadings. However, at higher surface loading, the growth rate was significantly dependent upon the type and charge of the headgroup. Dodecyl trimethyl ammonium chloride (DTAC) caused a greater increase in growth rate than sodium dodecyl sulfate (SDS) or dodecyl dimethyl ammonium propane sulfonate (DDAPS). With charge suppressed by the addition of salt, DDAPS showed the highest growth rate. Particle-image velocimetry (PIV) has been used to characterize the microstreaming around a bubble in aqueous solutions of these surfactants. Results indicate an enhancement of microstreaming velocities in the vicinity of the bubble when surfactant of any form is present in the solution, with the greatest enhancement for the surfactant with the most bulky headgroup. A clear difference could also be observed when bubbles underwent surface mode oscillations that dramatically increased the streaming velocities and resulted in a chaotic flow. The enhancement in surface oscillations and microstreaming was greatest for DTAC reflecting the bulkiness and charge of the headgroup. The resistance to mass transfer also played a role in enhancing the rectification of gas into the bubble. Surfactants with a longer chain length provide greater mass transfer resistance, and this contributes to higher growth rates.
1. INTRODUCTION It has been shown that surfactants enhance the rate of growth of a bubble in aqueous solutions via rectified diffusion in an acoustic field.1 3 One hypothesis for this increase in growth rate is that surfactants induce increased microstreaming. Elder4 reported that when surface active agents were added to water, a thin film would form to create a no-slip boundary condition on the bubble surface. This effect on the boundary layer increased the microstreaming until it was “broken up” by bubble pulsations. Gould5 has shown that the presence of microstreaming can significantly enhance the rectified diffusion rate. Microstreaming produces a flow in the vicinity of the bubble that enables fresh solution with higher gas concentration to be transported to the interface, thereby enhancing the mass transfer driving force. However Gould5 only observed significant enhancement when surface oscillations on the bubble were observed. Boon et al.6 have shown that at higher forcing amplitudes, microstreaming flows become more chaotic and greatly enhance mass transfer. Crum2 later offered the hypothesis that the presence of small amounts of surfactants could induce microstreaming behavior around the bubble even when surface oscillations were not observed. It was claimed that such streaming would increase r 2011 American Chemical Society
the growth rates, independent of acoustic driving pressure and bubble radius, without any distinct threshold for inception. However, no direct experimental evidence was provided to support this claim. There is also evidence to suggest that the surfactants at the boundary layer provide a level of resistance to the mass transfer. The surfactant layer may lower the flux of gas across the interface by physically impeding the gas molecules. In a dynamic situation such as an oscillating bubble undergoing rectified diffusion, this resistance can dramatically alter the difference in flux during the bubble expansion and compression. During bubble compression, the surfactant density increases, and the resistance to mass transfer across the interface rises, restricting gas diffusion out of the bubble. During bubble expansion, the surfactant density and hence resistance becomes lower, enabling a higher flux of gas into the bubble. The net result is an enhancement of gas accumulation in the bubble. Received: September 14, 2011 Revised: October 21, 2011 Published: November 02, 2011 24310
dx.doi.org/10.1021/jp208862p | J. Phys. Chem. C 2011, 115, 24310–24316
The Journal of Physical Chemistry C
ARTICLE
Table 1. Basic Properties of Surfactants Investigated
Mansfield7 has observed that expansion of surfactant films enables more evaporation through the interface than when it is compressed. If the film allows more mass transfer into the bubble during the expansion phase of an oscillation than that which diffuses out during compression, then rectification of gas as described above would occur. An analysis of the effect of resistance to mass transfer in the presence of surfactants has been presented by Fyrillis and Szeri.8 If indeed resistance to mass transfer affected the enhancement in growth by rectified diffusion, then the surfactant chain length should play a role, as it is known that the chain length of a surfactant can affect the degree of interfacial resistance.9 In our prior work, we showed that the enhancement in growth rate is indeed dependent on the type of surfactant and the density of surfactant loading on the bubble surface.3 We showed that the addition of 0.1 M sodium chloride electrolyte to sodium dodecyl sulfate (SDS) resulted in an enhanced growth rate at lower bulk concentrations of the surfactant due to enhanced surface loadings, which may have restricted mass transfer. The results suggested that the charge on the surfactant headgroup was not significant since growth rates for equivalent surface loadings of different surfactants were similar. We also observed that at high surfactant concentrations, an extremely rapid growth rate was observed. This rapid growth rate was attributed to an early onset of surface oscillations that likely produced microstreaming in the vicinity of the bubble. However, it was unknown as to why one surfactant, dodecyl trimethyl ammonium chloride (DTAC), showed such a response yet SDS did not. We suggested that the reason for the difference between SDS and DTAC could be due to the “bulkiness” of the headgroup. To validate these observations more broadly, we have now extended this study to include a number of other surfactants with
a wider range of properties. The effect of the addition of salt to DTAC is also studied in order to compare the results to those observed earlier with SDS + salt systems. We relate the rectified diffusion growth results obtained for various surfactants with that of microstreaming caused by cavitation. We use microscopic particle-image velocimetry (PIV) to visualize the acoustic streaming velocities around bubbles in the presence of various surfactants. We aim to explore the effect of surfactant chain length and headgroup charge on both rectified diffusion and acoustic microstreaming velocities around a bubble.
2. EXPERIMENTAL DETAILS The surfactants used were of the purest grades available: SDS (VWR International, purity >99%), DTAC (TCI Japan, purity >99%), dodecyl dimethyl ammonium propane sulfonate (DDAPS; Sigma, purity >97%), sodium 1-pentane sulfonate (Alfa Aesar, purity >99%, 99%). Sodium chloride was supplied by Merck Germany (purity >99.5%). All solutions were made up in Milli-Q water (Millipore 0.22 μm pore size, conductivity
