Coupling Effects Of Surface Elasticity And Disjoining Pressure On Film Drainage Process

Foam is widely used in industry and life.In the foam system,the dispersed gas phase is separated by the liquid film,thus the liquid film between the foams can be partially broken by the liquid drainage to control the stability of the foam.Therefore,the study on the stability of liquid film drainage is helpful to reveal the decay mechanism of foam and thus establish a theoretical foundation for the efficient application of foam in various fields.Previous studies mainly focused on the experimental aspect,or only considered a single influencing factor,and lacked a profound theoretical understanding.Therefore,In this paper,a vertical liquid-film drainage model including the thickness of the liquid film,surface velocity,surface and bulk concentration of the surfactant was established based on the insoluble and soluble surfactant of the liquid film respectively.The influences of disjoining pressure,surface elasticity,solubility and adsorption coefficient on the drainage process are analyzed.The main contents are as follows:(1)For the vertical liquid film drainage process with soluble surfactant,exponential disjoining pressure model and surface elastic equation are introduced.A set of evolution equations including liquid film thickness,surface velocity,surface and bulk surfactant concentration,is established based on the lubrication theory.(2)Aiming at the vertical liquid film drainage process with insoluble surfactant,the calculation model was simplified by considering the coupling effect of surface elasticity and disjoining pressure.The evolution characteristics of liquid film under the separate and coupled effects of surface elasticity and disjoining pressure were analyzed by using Freefem finite element analysis software for numerical calculation.The results show that increasing the surface elasticity in the early stage of drainage can increase the thickness of the liquid film,reduce the surface velocity and promote the liquid countercurrent,so as to slow down the process of drainage.After the occurrence of black film in the later stage,the electrostatic repulsion force in the separation pressure plays a major role in delaying the "ripening" of the liquid film.(3)In view of the vertical liquid film drainage process with soluble surfactant,the influence of solubility coefficient,adsorption coefficient and limiting dilational elasticity on the characteristics of liquid film drainage was analyzed on the premise of considering the effect of disjoining pressure.The results showed that the stability of the liquid film increased with the decrease of the solubility and adsorption coefficient,and the effect of the limting dilational elasticity changed with the drainage process.