Zheng Qianlu, Hawthorne Nathaniel, Batteas James D, Espinosa-Marzal Rosa M
Department of Civil and Environmental Engineering, University of Illinois at Urbana─Champaign, Urbana, Illinois 61801, United States.
Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States.
Langmuir. 2024 Feb 9. doi: 10.1021/acs.langmuir.3c03519.
Ionic liquids (ILs) are a promising class of lubricants that allow dynamic friction control at electrified interfaces. In the real world, surfaces inevitably exhibit some degree of roughness, which can influence lubrication. In this work, we deposited single-layer graphene onto 20 nm silica nanoparticle films to investigate the effect of surface curvature and electrostatic potential on both the lubricious behavior and interfacial layering structure of 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide on graphene. Normal force and friction force measurements were conducted by atomic force microscopy using a sharp silicon tip. Our results reveal that the friction coefficient at the lubricated tip-graphene contacts significantly depends on surface curvature. Two friction coefficients are measured on graphene peaks and valleys with a higher coefficient measured at lower loads (pressures), whereas only one friction coefficient is measured on smooth graphene. Moreover, the electrotunability of the friction coefficient at low loads is observed to be significantly enhanced in peaks and valleys compared with smooth graphene. This is associated with the promoted overscreening of surface charge on convex interfaces and the steric hindrance at concave interfaces, which leads to more layers of ions (electrostatically) bound to the surface, i.e., thicker boundary films (electrical double layers). This work opens new avenues to control IL lubrication on the nanoscale by combining topographic features and an electric field.
离子液体（ILs）是一类很有前景的润滑剂，能够在带电界面实现动态摩擦控制。在现实世界中，表面不可避免地会呈现出一定程度的粗糙度，这可能会影响润滑效果。在这项工作中，我们将单层石墨烯沉积在20纳米的二氧化硅纳米颗粒薄膜上，以研究表面曲率和静电势对1-乙基-3-甲基咪唑双（三氟甲基磺酰）亚胺在石墨烯上的润滑行为和界面分层结构的影响。使用尖锐的硅探针通过原子力显微镜进行法向力和摩擦力测量。我们的结果表明，润滑的探针-石墨烯接触处的摩擦系数显著取决于表面曲率。在石墨烯的峰谷处测量到两个摩擦系数，在较低负载（压力）下测量到的系数较高，而在光滑的石墨烯上只测量到一个摩擦系数。此外，与光滑石墨烯相比，在峰谷处观察到低负载下摩擦系数的电可调性显著增强。这与凸界面上表面电荷的过屏蔽增强以及凹界面处的空间位阻有关，这导致更多层离子（静电）结合到表面，即更厚的边界膜（双电层）。这项工作通过结合地形特征和电场，为在纳米尺度上控制离子液体润滑开辟了新途径。
