Spreading Of A Surfactant-laden Drop On Thin Liquid Film

The liquid drop of surfactant solutions will spread into thin film when deposited on the solid substrate or precursor. Such flows that are driven by so-called interfacial Marangoni stresses are of great importance in a wide range of industrial and biomedical applications including petrochemical engineering, magnetofluid preparation, drying of semiconductor wafers in the microelectronics industry and surfactant replacement therapy for neonates. The spreading of surfactant solutions has attracted considerable interest in the field of fluid mechanics, colloid and interface science both theoretically and experimentally. It is an advancing topic of investigation with subject intersection.The spreading of surfactant laden-drops on thin liquid film in presence of interfacial heating or disjoining pressure are investigated thoroughly and intensively in the thesis, combining the method of model derivation and numerical simulation. The main parts of the thesis are as following:(1) For the spreading of drop containing insoluble surfactant driven by thermal and concentration gradients, use of lubrication theory yields two coupled sets of partial differential equations for the film thickness and surfactant surface concentration on conditions of evaporation and no evaporation, respectively. Both coupled equations are numerical simulated for different heating conditions, uniform and non-uniform. The evolution characteristics and the effects of dimensionless parameters on the spreading process are analyzed, including Marangoni parameter, air-liquid Biot number and surface Peclet number, and interfacial thermal resistance, vapor recoil number and evaporation number for evaporation condition. It is shown that thermocapillary and Marangoni stress are main driving force,which are influenced by the above parameters leading to variations of the spreading process.(2) For the spreading of drop containing soluble surfactant driven by thermal and concentration gradients, use of lubrication theory yields two coupled sets of partial differential equations for the film thickness, surfactant concentrations on the surface and in the bulk on conditions of evaporation and no evaporation, respectively. Both coupled equations are numerical simulated only for uniform heating conditions. The evolution characteristics and the effects of dimensionless parameters on the spreading process are analyzed, including solubility parameter, dimensionless desorption rate constant and bulk Peclet number. It is shown that sorptive flux between surface monomer and bulk monomer of surfactant weakened the stabilizing effect of Marangoni stress,which leads to unstable aspects in the spreading of soluble surfactant.(3) For the spreading of a drop containing high concentration of soluble surfactant, which is beyond the CMC, in the presence of thermal effect, use of lubrication theory yields two coupled sets of partial differential equations for the film thickness, surfactant monomer concentrations on the surface and in the bulk and micelle concentration on conditions of evaporation and no evaporation, respectively. Both coupled equations are numerical simulated only for uniform heating conditions. The evolution characteristics and the effects of dimensionless parameters on the spreading process are analyzed, including dimensionless surfactant mass, aggregation constant, micelle size, bulk kinetics parameter and micelle Peclet number. It is shown that the creation of micelles or the breakup of micelles result in the different spreading region dominated by different driving force, either thermocapillary or Marangoni stress,which leads to distinct features in the spreading, such as appearance of secondary leading front.(4) For the spreading of thin film containing insoluble surfactant under the disjoining pressure, a universal model is constructed on the base of experiment data by considering the influence of surfactant features and concentration on the intermolecular attraction force and repulsion force. The equation of free energy is also presented. The dependences of attraction/repulsion ratio, disjoining pressure and free energy are analyzed due to the variation of surfactant concentration and film thickness. A coupled set of partial differential equations for the film thickness and surfactant surface concentration under disjoining pressure is derived. The results of numerical simulations are presented.