Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States.
Center for Complex Fluids Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States.
Langmuir. 2021 Mar 23;37(11):3309-3320. doi: 10.1021/acs.langmuir.0c03348. Epub 2021 Mar 9.
When an insoluble surfactant is deposited on the surface of a thin fluid film, stresses induced by surface tension gradients drive Marangoni spreading across the subphase surface. The presence of a predeposited layer of an insoluble surfactant alters that spreading. In this study, the fluid film was aqueous, the predeposited insoluble surfactant was dipalmitoylphosphatidylcholine (DPPC), and the deposited insoluble surfactant was oleic acid. An optical density-based method was used to measure subphase surface distortion, called the Marangoni ridge, associated with propagation of the spreading front. The movement of the Marangoni ridge was correlated with movement of surface tracer particles that indicated both the boundary between the two surfactant layers and the surface fluid velocities. As the deposited oleic acid monolayer spread, it compressed the predeposited DPPC monolayer. During spreading, the surface tension gradient extended into the predeposited monolayer, which was compressed nonuniformly, from the deposited monolayer. The spreading was so rapid that the compressed predeposited surfactant could not have been in quasi-equilibrium states during the spreading. As the initial concentrations of the predeposited surfactant were increased, the shape of the Marangoni ridge deformed. When the initial concentration of the predeposited surfactant reached about 70 A/molecule, there was no longer a Marangoni ridge but rather a broadly distributed excess of fluid above the initial fluid height. The nonuniform compression of the annulus of the predeposited monolayer also caused tangential motion ahead of both the Marangoni ridge and the boundary between the two monolayers. Spreading ceased when the two monolayers reached the same final surface tension. The final area per molecule of the DPPC monolayer matched that expected from the equilibrium DPPC isotherm at the same final surface tension. Thus, at the end of spreading, there was a simple surface tension balance between the two distinct monolayers.
当不溶性表面活性剂沉积在薄流体膜的表面上时，表面张力梯度引起的应力会驱动马兰戈尼（Marangoni）在亚相表面上扩展。预先沉积的不溶性表面活性剂层的存在会改变这种扩展。在这项研究中，流体膜为水性，预先沉积的不溶性表面活性剂为二棕榈酰基磷脂酰胆碱（DPPC），沉积的不溶性表面活性剂为油酸。基于光密度的方法用于测量与扩展前沿传播相关的亚相表面变形，称为马兰戈尼脊。马兰戈尼脊的运动与表面示踪粒子的运动相关联，这些粒子既指示了两层表面活性剂之间的边界，也指示了表面流体的速度。随着沉积的油酸单层扩展，它压缩了预先沉积的 DPPC 单层。在扩展过程中，表面张力梯度延伸到预先沉积的单层中，该单层从沉积的单层不均匀地压缩。扩展速度非常快，以至于在扩展过程中，压缩的预先沉积的表面活性剂不可能处于准平衡状态。随着预先沉积的表面活性剂初始浓度的增加，马兰戈尼脊的形状发生了变形。当预先沉积的表面活性剂的初始浓度达到约 70 A/分子时，不再有马兰戈尼脊，而是在初始流体高度上方有一个广泛分布的流体过剩。预先沉积的单层环的不均匀压缩也导致了马兰戈尼脊和两层之间边界的前方的切向运动。当两层到达相同的最终表面张力时，扩展停止。最终 DPPC 单层的每个分子的面积与相同最终表面张力下平衡 DPPC 等温线预期的面积相匹配。因此，在扩展结束时，两层之间存在简单的表面张力平衡。
