Superspreading

Aqueous dispersions of certain trisiloxane polyoxyethylene surfactants spread rapidly over hydrophobic surfaces, making them useful in water-based coatings for plastics. Several features of this spreading challenge existing models which were developed for single-component fluids. Recent work has demonstrated four particular features which demand significant further development of theory. First, inital inertial spreading is followed by a period in which the area increases linearly with time. Second, the duration of the linear period and the limiting area are proportional to surfactant concentration. Third, there are maxima in the spreading rate versus the surfactant concentration and the substrate surface energy implying competitive processes. Fourth, vesicle-forming surfactants appear to spread faster than comparable micelle-forming examples, suggesting that surfactant transport is rate controlling and is dependent on surfactant microstructures in ways that have not been anticipated. In addition, unlike the spreading of surfactant solutions over water-wettable surfaces such as clean glass, dendritic fingering of the spreading edge is not a prominent feature. In the past year, it has been demonstrated that these characteristics are also exhibited by certain nonionic and ionic hydrocarbon surfactants and thus are general features of the spreading of surfactant solutions on surfaces that are not water-wettable. The trisiloxanes are still the only surfactants that promote spreading of aqueous solutions over surfaces as hydrophobic as polyethylene.