Adsorption dynamics and fluid mechanics of surfactant solutions
Adsorption dynamics of surfactants at air/water interface is important for many industrial applications, such as foaming, coating flows, lung surfactants, and drop or jet breakup. In order to control or predict the dynamic properties in these applications, it is necessary to understand and describe qualitatively and quantitatively the dynamic behaviors of free surface flows in the presence of surfactants. Rigorous and robust numerical algorithms based on Galerkin finite element (FE) method are developed for predictions of dynamic adsorption and hydrodynamics in accord with moving boundaries. The presence of surfactant results in surface tension gradients which affect the breakup dynamics of liquid bridges and drop formation. Effects of various parameters, such as viscosity and surface activity of surfactants, on breakup dynamics are elucidated. The accuracy of computational predictions is supported by demonstrating that computed results accord well with high-speed visualization experiments and existing scaling theories. Hydrodynamic effects on dynamic surface tension (DST) measurement in pulsating bubble surfactometer (PBS) are also examined theoretically by using similar FE algorithm. The assumption that hydrodynamic effects play a negligible role in the requisite balance of forces at the bubble surface is shown to be valid for currently commercial available setup, which operates at a low frequency (<2 >Hz). Predictions from hydrodynamic models are supported by direct visualization of bubble shapes and particle imaging velocimetry (PIV) measurements of flow fields in the liquid surrounding the bubbles. DST measurement at high frequency range is also investigated. Inertial force and non-uniform surface tension lead to erroneous apparent DST in high frequency range (>10 Hz). Remedies for these errors are provided in this thesis for potential improvements in the apparatus design for fast DST measurements. ^
Major Professors: Osman A. Basaran, Purdue University, Elias I. Franses, Purdue University.