Surfactant adsorption and Marangoni flow in liquid jets. I. Experiments.

The adsorption of surfactants at an expanding liquid surface has been studied in a gravity-driven laminar water jet with Reynolds numbers in the range from 1000 to 2000. Surface concentrations of hexadecyltrimethylammonium bromide (C(16)TAB) were deduced from ellipsometric measurements, using a calibration made previously with neutron reflection. Simultaneous measurements of the velocity profile within the jet were made with laser Doppler velocimetry. These two noninvasive techniques were able to measure conditions to within 1 mm of the nozzle, where rates of surface expansion were as high as 300 s(-1). For the laminar jet without surfactant, the measurements are in excellent agreement with CFD calculations and with the theoretical result that the surface velocity varies as z(1/3), where z is the distance from the nozzle. Close to the nozzle the high rate of surface expansion drives both rapid diffusional transport to the surface, and rapid convection on the surface, resulting in a low concentration of surfactant. Higher concentrations of surfactant downstream cause a Marangoni stress which decelerates the surface-an effect clearly shown by the velocity data. In the presence of 0.2 M salt, which significantly depresses the cmc, the adsorption of C(16)TAB is greatly reduced, probably because it forms cylindrical micelles, which diffuse much more slowly than free monomers. The apparatus is shown to be a very suitable platform for investigating surfactant adsorption and Marangoni flows under carefully controlled hydrodynamic conditions.