Kinetics of Spreading of Surfactant Solutions
Two different mechanism of influence of surfactants on hydrodynamics of spreading are considered: (i) the presence of an inhomogeneous distribution of surfactants on liquid-air interfaces, which results in surface tension gradients, which in their turn cause tangential stresses and Marangoni flow, and (ii) a slow spreading of surfactant solutions over hydrophobic substrates, which is caused by adsorption of surfactant molecules on a bare hydrophobic interface in front of the moving three phase contact line. In section 2 we present results of the theoretical and experimental study of the spreading of an insoluble surfactant over a thin liquid layer. Initial concentrations of surfactant above and below critical micelle concentration (CMC) is considered. If the concentration is above the CMC two distinct stages of spreading are found (a) the fast first stage, which is connected with the micelles dissolution; (b) the second slower stage, when the surfactant concentration becomes below CMC over the whole liquid surface. During the second stage, the formation of a dry spot in the centre of the film is observed. A similarity solution of the corresponding equations for spreading results in the good agreement with the experimental observations. In section 3 the spreading of aqueous surfactant solutions over hydrophobic surfaces is considered from both theoretical and experimental points of view. Aqueous droplets do not wet a virgin solid hydrophobic substrate and do not spread, however, surfactant solutions spread. It is shown that the transfer of surfactant molecules from the aqueous droplet onto the hydrophobic surface changes the wetting characteristics in front of the droplet on the moving three phase contact line. The adsorption of surfactant molecules results in an increase of the solid-vapour interfacial tension and hydrophilisation of the initially hydrophobic solid substrate in front of the spreading droplet. This process causes aqueous droplets to spread over time. The time evolution of the spreading of aqueous droplets is predicted and compared with experimental observations. The assumption that surfactant transfer from the droplet surface onto the solid hydrophobic substrate controls the rate of spreading is confirmed by our experimental observations.
KeywordsContact Angle Critical Micelle Concentration Surfactant Concentration Surfactant Solution Surfactant Molecule
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