Fundamentals of acoustic cavitation: the effect of surfactants

© 2012 Thomas Seak Hou LeongIn this thesis, the role of surfactants in enhancing the growth rate of bubbles in an ultrasound field by the process known as rectified diffusion has been investigated. The physical properties of a surfactant, such as its head group, chain length and charge, are shown to influence the enhancement in growth rate. The enhancement is shown to be due to several contributing effects. The resistance to mass transfer provided by a layer of surfactant around the bubble is found to be one cause of enhanced rectification. During bubble oscillations, an imbalance in resistance to mass transfer between the maximum and minimum radius occurs, that renders mass transfer to be lower during bubble expansion as the density of surfactant packing decreases and mass transfer to be higher during bubble compression as the density of surfactant packing increases. Numerical simulations are used to show that the resistance to mass transfer can bring existing theoretical predictions closer to the experimentally obtained results for surfactant solutions. Surfactants are also shown to influence the microstreaming velocities of the flow streams of fluid around an oscillating bubble, even in the absence of observable surface oscillations. Previously, researchers have shown that enhanced microstreaming fluid velocities increase the growth rate of a bubble, due to faster renewal of solution with higher gas concentration to the mass transfer interface. In the current study, very strong enhancement of microstreaming is only measured when unstable surface oscillations are observed. The effect of the surfactant viscoelasticity is shown using numerical simulations to have a significant influence on the stability of the oscillating bubble surface. At low acoustic driving pressure, the surface dilatational viscosity of a surfactant lowers the stability. Such calculations are consistent with the experimental observation of some surfactants at high bulk concentrations. These calculations also suggest that the reason for enhanced microstreaming flows is due to the surface being less stable in the presence of surfactants at low driving pressures. At high driving pressures, the tendency is reversed, and the surfactant viscoelasticity dramatically reduces the oscillation amplitude which in turn serves to damp instability. While this thesis has clarified a range of issues associated with single bubble behaviour, it concludes that the translation of these results to multibubble systems remains a challenge