Surfactant transport in Marangoni flow

Spreading due to a surface tension gradient, known as Marangoni spreading, is common in natural and technological settings. The surface tension gradient can arise from the localized deposition of a surface-active material on a liquid subphase. Much of the literature focuses on spreading on a thin, viscous subphase, where the lubrication approximation can be utilized. However, spreading on deep, low viscosity subphases, where the lubrication approximation does not hold, are less studied. The focus of this thesis is understanding Marangoni spreading on such deep, low viscosity subphases. In this regime, both capillary waves and Marangoni flows form. Further, this thesis shows how changing key parameters impacts the Marangoni flows and capillary waves.

Changes of surfactant properties, such as the solubility, desorption rate, and concentration, impacts the pathways the surfactant molecule can take or the rate in which it traverses through a particular pathway. As we change the Marangoni stresses by varying the surfactant properties, the amplitude of the capillary waves also changes. However, in the cases we examined, the speed of the capillary wave remains the same. As spreading progresses, a surface distortion arising from the Marangoni stresses separates from the slowest moving capillary wave. The movement of the surfactant front is also impacted by varying initial surfactant parameters.

Varying the initial surface tension of the subphase by pre-depositing insoluble surfactant monolayer also impacts the spreading. The presence of a pre-deposited monolayer changes how the deposited surfactant is transported on the subphase surface. The non-uniform compression of the pre-deposited monolayer causes the surface tension gradient to extend past the deposited surfactant front and into the pre-deposited monolayer, impacting the stresses driving spreading. The overall surface deformation driven by the Marangoni flow is altered – to the point where the slowest moving surface peak disappears at high pre-deposited surfactant concentration. In addition, the slowest moving peak speed is dependent on the pre-deposited surfactant concentration. As the concentration of the pre-deposited surfactant increases, the speed decreases monotonically.

The roughness of the substrate and the thickness of the subphase also impacts spreading. Preliminary work was performed on surfaces where the roughness was formed by grooves. With the grooves present, there are two subphase depths: one at the highest point of the surface between the grooves and one at the lowest point in the grooves. The presence of the grooves, with a width on the order of the capillary length of the aqueous subphase, did not detectibly impact the surfactant front movement, which is driven by the Marangoni stresses. However, the grooves visibly impacted dewetting of the substrate, which is governed by recirculation flows in the subphase. The coupling of the flows in and beside the grooves alters the recirculation flows and the dewetting behavior. The grooves themselves never dewetted, while between the grooves, at the highest point of the substrate, it was possible for the substrate to dewet, depending on the initial subphase thickness.