(64a) Synergistic Enhancement of Marangoni Flows Driven By Surface Gradients of Binary Catanionic Surfactant Mixtures

Marangoni flows are driven by surface tension gradients along a fluid interface and influence a variety of technologies and natural processes. For example, they influence the uniformity of coating films, post-deposition dispersal of aerosol medications, cleaning of microfabricated devices and the propulsion of some insects on water surfaces. This work focuses on solutal Marangoni transport driven by localized deposition of surfactants at the air/water interface, and its capacity to transport particles pre-deposited on the surface. When a drop of surfactant solution is deposited at the surface of an initially surfactant-free aqueous subphase, surfactants spread out radially, and the associated radial Marangoni flow entrains particles present on the surface. In general, the speed and extent of this Marangoni spreading depend on the spreading parameter (the surface tension difference between the surfactant solution drop and the clean subphase), surfactant inventory (dictated by surfactant concentration and drop volume), subphase thickness, the surface tension equation of state, and the surfactant’s transport properties. For binary surfactant mixtures, the concept of surface tension synergism is well-established. It describes the condition where the same surface tension reduction is achieved by a binary surfactant mixture at a lower total concentration than that required for either of the single-surfactant counterparts. Equivalently, the binary mixture produces a lower surface tension at the same total concentration as either single-surfactant counterpart. We propose a novel concept of Marangoni spreading synergism, which describes the condition where Marangoni flow either occurs at a greater maximum velocity or persists over a greater distance after spreading a drop of a binary surfactant mixture compared to either single-surfactant system at the same total concentration. Marangoni spreading synergism differs from the well-established concept of surface tension synergism in that the latter is a purely thermodynamic phenomenon while the former is a dynamic transport process that depends not only on the surface tension equation of state but also on transport properties. The objective of this work is to test whether Marangoni spreading synergism occurs and the degree to which it is correlated with surface tension synergism.

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.