(543c) Alteration of Marangoni Flow in the Presence of a Pre-Deposited Insoluble Surfactant
AIChE Annual Meeting
Friday, November 20, 2020 - 8:15am to 8:30am
As long as the surface pressure of the pre-deposited monolayer is less than that of the deposited surfactant monolayer, spreading proceeds until the surface pressures of the unmixed endogenous and exogenous surfactant monolayers are equal. Oleic acid spreads to form a monolayer, with residual oleic acid forming small lenses at the air/water interface. Since the equilibrium surface pressure of an oleic acid monolayer is fixed, the final surface pressure and thus the final area per DPPC molecule in the compressed endogenous monolayer is the same in every case. The final area per DPPC molecule is fixed at the area per molecule corresponding to the surface pressure of an oleic acid monolayer, independent of its starting surface concentration. The Marangoni flow and resulting surface deformation are altered by increasing the initial pre-deposited surfactant surface concentration. Unlike Marangoni flow with no pre-deposited surfactant, there is surface flow ahead of the Marangoni ridge as the pre-deposited monolayer is compressed. Due to the compression of the pre-deposited monolayer, the surface tension gradient now extends beyond the deposited monolayer into the pre-deposited monolayer. The change in the surface tension gradient causes normal and radial velocities to extend into the region occupied by the pre-deposited monolayer. This is in sharp contrast to surfactant spreading in the absence of pre-deposited surfactant, where no normal or radial velocity components precede the deposited exogenous surfactant front.