SUPERSPREADING OF AQUEOUS FILMS CONTAINING TRISILOXANE SURFACTANT ON MINERAL-OIL

The spreading of water droplets, containing various surfactants, on li quid mineral oil surfaces has been investigated. Only ''superspreading '' trisiloxane surfactants were observed to promote rapid spreading on mineral oil, and the spreading characteristics of these systems diffe r substantially from those observed on solid substrates of comparable hydrophobicity. These differences include significantly faster spreadi ng rates and monotonically increasing spreading rates with increasing surfactant concentration. Super-spreading is attributed to Marangoni n ow along the mineral oil-water interface. The extremely fast spreading on the mineral oil surface, as compared to hydrophobic solid surfaces , is attributed to the absence of the no-slip condition at the mineral oil-water interface. Real-time videomicroscopy of the spreading of aq ueous droplets containing the trisiloxane surfactant M(D'E4OH)M reveal ed stepwise motion of the leading edges of the aqeuous drop driven by disintegration of large surfactant aggregates at the mineral oil-water interface. This observation suggests that disintegrating aggregates i nstantaneously deliver large amounts of surfactant to the mineral oil- water interface, creating large surface tension gradients that are req uired for Marangoni flow. These results imply an important role of agg regate disintegration during the spreading of surfactant dispersions o n hydrophobic solid substrates.