The Interfacial Physics Group

Carnegie Mellon University, Department of Physics

Surfactant Self-Assembly Near Contact Lines:
Control of Advancing Surfactant Solutions

by Benjamin Frank



Surfactant self-assembly at the liquid-vapor, solid-liquid, and solid-vapor interfaces controls the wetting behavior of advancing surfactant solutions. While different surfactants exhibit different static and dynamic wetting properties, we show that these behaviors can be understood through an examination of microscopic structures driven by surfactant-surface interactions. We show that surface concentration gradients and solution mobility are required for dendritic spreading, which occurs when an ionic surfactant spreads on a surface of the same charge. When the surfactant has the opposite charge as the ionized substrate, it can strongly adsorb to the surface ahead of the advancing solution, forming barriers to fluid advance. In this case, the contact line advances by jumping forward along the surface. At low concentrations, the contact line then retracts, exhibiting autophobic behavior. Surfactant with an intermediate attraction to the surface begins to advance by fingering. However, inhibition of the fluid advance by surfactant adsorbtion at the contact line alters the dendritic pattern and can change the spreading to stick-jump motion. By tuning the strenth of the surfactant-surface interaction, we have observed a spectrum of spreading behaviors from dendritic to stick-jump. We have directly observed the development of the surfactant barriers at the contact line which cause the autophobic and stick-jump behaviors. Different macroscopic spreading behavior and microscopic development of the self-assembly occur above and below bulk solution concentration of ~0.45 cmc (critical micellar concentration). We put an upper limit on the size of the region on the solid-vapor interface relevant in determining the contact angles.