Spreading dynamics of surfactant solutions

The spreading behavior of microdrops of surfactant solutions at solid surfa ces has been studied. The influence on the spreading of different factors, such as the drop lifetime prior to surface contact, surface tension dynamic s, surface energy, and surfactant properties, was systematically investigat ed. The results obtained suggest the existence of two spreading regimes exh ibiting different spreading characteristics: In the first, nondiffusive reg ime, the spreading is very rapid and controlled to different extents by ine rtia, gravity, and capillarity, depending on the drop size, impact energy, and interfacial tension balance. It is shown in this study that the initial drop surface tension, which is set by the surface tension decay rate and t he drop lifetime prior to the surface impact, strongly influences the maxim um spreading distance in the nondiffusive spreading regime. The second, dif fusion-controlled regime, is characterized by slower concentration-dependen t spreading rates. The spreading rate is, here, mainly controlled by the di ffusive transport of surfactant to the expanding liquid-vapor interface. In this regime, the drop base radius exhibits an approximate r(b)(2) proporti onal to t dependence on time. The spreading kinetics at hydrophobic surface s has been discussed in the framework of a simplified theory. Depending on the assumptions regarding the drop shape, r(b)(5/2) proportional to t to r( b)(2) proportional to t Spreading laws are obtained. This agrees reasonably with the experimentally observed relationship.