(64a) Synergistic Enhancement of Marangoni Flows Driven By Surface Gradients of Binary Catanionic Surfactant Mixtures
AIChE Annual Meeting
Monday, November 8, 2021 - 8:00am to 8:15am
To experimentally search for Marangoni spreading synergism, anionic sodium octyl sulfate (SOS), cationic octyl trimethylammonium bromide (OTAB) and their binary catanionic mixtures were used to perform the spreading experiments. This particular system was chosen as it is known to exhibit a strong surface tension synergism, similar to other catanionic surfactant pairs due to strong mutual attractions. All solutions were prepared in purified water. The surface tension synergism was verified in this work by measuring the surface tension isotherms of both single-surfactant and binary mixture solutions with pendant drop tensiometry. The Marangoni spreading was conducted by gently pipetting a drop of 5 µL surfactant solution on a 4.8 mm-thick subphase of pure water in a Petri dish. The subphase was carefully sprinkled with dilute talc particles as tracers before spreading. Using a camera with 120 fps frame rate, the radial spreading of talc particles was captured from a top view during the spreading event and the particle spreading radius was recorded as a function of time. In general, spreading of the talc particles sweeps an expanding circular region around the drop deposition site that is free of particles. The radius of this swept region was monitored as a function of time. The surface tracer particles rapidly accelerate to pass through a maximum speed, eventually slowing until spreading ceases. Results show that, over a broad range of compositions tested (from 0.05 to 0.95 of SOS fraction), the binary SOS/OTAB solutions at fixed total concentrations of 1, 5 and 10 mM produce significantly faster Marangoni spreading velocities and larger maximum extents of spreading radius than both single-surfactant systems at the corresponding concentrations. This verifies the existence of Marangoni spreading synergism. At the concentrations and SOS/OTAB compositions considered thus far, the system exhibits both Marangoni and surface tension synergism. Current experimental work is focused on broadening solution conditions to identify the limits, if they exist, of where Marangoni and surface tension synergism diverge. Briefly, we note that another catanionic mixture that does yield surface tension synergism, sodium dodecyl sulfate and tetradecyl trimethylammonium bromide, did not exhibit Marangoni synergism. Experimental work is complemented by ongoing numerical modeling of the spreading process to identify how transport and thermodynamic properties combine to dictate the strength of Marangoni synergism.