Department of Chemical and Biomolecular Engineering, Tulane University , New Orleans, Louisiana 70118, United States.
Department of Tropical Medicine, School of Public Health and Tropical Medicine, Tulane University , New Orleans, Louisiana 70112, United States.
Langmuir. 2018 Jan 23;34(3):801-806. doi: 10.1021/acs.langmuir.7b03023. Epub 2017 Oct 13.
We present an exploratory study of the tribological properties between an AFM probe and a Au(111) surface in an aqueous environment while subjected to applied surface potentials. Using a three-electrode setup, the electrical potential and interfacial electric field on a Au(111) working electrode are controlled. Lateral force microscopy is used to measure the friction forces between the AFM probe and the Au surface. As the AFM probe approaches the surface, normal forces are also measured to gain insight into the interfacial forces. When a positive potential is applied to the Au surface, the friction is found to rise sharply at a critical potential and level off at a relatively high value. However, when a negative potential is applied, the friction forces are low, even lower compared to the open circuit potential case. These changes in friction, by a factor of approximately 35, as a function of the applied potential are found to be reversible over multiple cycles. We attribute the origin of the high friction at positive potentials to the formation of a highly confined, ordered icelike water layer at the Au/electrolyte interface that results in effective hydrogen bonding with the AFM probe. At negative potentials, the icelike water layer is disrupted, resulting in the water molecules acting as boundary lubricants and providing low friction. Such friction experiments can provide valuable insight into the structure and properties of water at charged surfaces under various conditions and can potentially impact a variety of technologies relying on molecular-level friction such as MEMs.
我们在施加表面电势的情况下，对 AFM 探针与 Au(111) 表面在水相环境中的摩擦学性质进行了探索性研究。通过三电极装置，控制 Au(111)工作电极的电势能和界面电场。利用横向力显微镜测量 AFM 探针与 Au 表面之间的摩擦力。当 AFM 探针接近表面时，也测量了法向力，以深入了解界面力。当对 Au 表面施加正电势时，发现摩擦力在临界电势处急剧上升，并在相对较高的值处稳定下来。然而，当施加负电势时，摩擦力较低，甚至比开路电势情况下还要低。这种摩擦变化，在施加的电势作用下，大约有 35 倍的变化幅度，在多个循环中是可逆的。我们将正电势下高摩擦力的起源归因于在 Au/电解质界面处形成高度受限、有序的类冰状水层，导致与 AFM 探针的有效氢键。在负电势下，类冰状水层被破坏，水分子充当边界润滑剂，提供低摩擦力。此类摩擦实验可以为在不同条件下带电荷表面上水的结构和性质提供有价值的见解，并可能影响依赖于分子级摩擦力的各种技术，如 MEMs。
