Dynamic Characteristics Of Droplet Evolution With Soluble Surfactant In Different Spreading Systems

Thin liquid film flows containing surfactant on solid substrate have attractedconsiderable attention in literatures for the widely applications in biomedical processing,aerospace engineering and chemical engineering industries. The film of thickness lessthan100nm is subject to disjoining pressure or conjoining pressure induced byintermolecular forces, which determine its stability and wettability. In addition, thedynamics of droplet spreading on preset films are also dominated by Marangoni stress,which forms under the action of non-uniform distribution of surface tension. Accordingto the relation between the evolution trend of surface tension gradient and the direction offilm flow, the systems of surfactant solutions are classified into positive, negative andneutral systems. In the present thesis, the dynamics and stability of droplet evolution indifferent systems are discussed via numerical simulation and theoretical analysis. Themain parts of thesis are as follows:(1) Considering the process of an ultrathin surfactant-laden droplet spreading onpreset film, the nonlinear partial differential evolution equations for the film thickness,surfactant interfacial and bulk concentrations are established based on the lubricationapproximation. The evolution process under the action of van der Waals force and Bornforce is simulated via PDECOL program, and the effects of intermolecular forces,collectively called disjoining pressure or conjoining pressure are analyzed.(2) Taking intermolecular forces into account, the spreading of droplet in positive,negative and neutral systems are simulated, and the influence of Marangoni effect on theflow is discussed. Additionally, the effects of disjoining pressure and conjoining pressureon the droplet evolution mechanism in the negative system are explored. Theproportions and relations of Marangoni stress, capillary force and intermolecularforces during the surfactant diffusion process are revealed via numerical simulation.(3) Based on the non-modal stability theory, the base state and disturbance evolutionequations for the film thickness, surfactant interfacial and bulk concentrations areestablished. Transient growth analysis (TGA) is carried out to investigate the stabilitycharacteristics of evolution process, and the effects of van der Waals force and Born forceare discussed, as well as the typical parameters.