Zhao Binyu, Wang Xingya, Song Yang, Hu Jun, Lü Junhong, Zhou Xingfei, Tai Renzhong, Zhang Xuehua, Zhang Lijuan
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China.
Phys Chem Chem Phys. 2015 May 28;17(20):13598-605. doi: 10.1039/c5cp01366f.
Micropancakes are quasi-two-dimensional micron-sized domains on crystalline substrates (e.g. highly oriented pyrolytic graphite (HOPG)) immersed in water. They are only a few nanometers thick, and are suspected to come from the accumulation of dissolved air at the solid-water interface. However, the exact chemical nature and basic physical properties of micropancakes have been under debate ever since their first observation, primarily due to the lack of a suitable characterization technique. In this study, the stiffness of micropancakes at the interface between HOPG and ethanol-water solutions was investigated by using PeakForce Quantitative NanoMechanics (PF-QNM) mode Atomic Force Microscopy (AFM). Our measurements showed that micropancakes were stiffer than nanobubbles, and for bilayer micropancakes, the bottom layer in contact with the substrate was stiffer than the top one. Interestingly, the micropancakes became smaller and softer with an increase in the ethanol concentration in the solution, and were undetectable by AFM above a critical concentration of ethanol. But they re-appeared after the ethanol concentration in the solution was reduced. Clearly the evolution and stiffness of the micropancakes were dependent on the chemical composition in the solution, which could be attributed to the correlation of the mechanical properties of the micropancakes with the surface tension of the liquid phase. Based on the "go-and-come" behaviors of micropancakes with the ethanol concentration, we found that the micropancakes could actually tolerate the ethanol concentration much higher than 5%, a value reported in the literature. The results from this work may be helpful in alluding the chemical nature of micropancakes.
微煎饼是浸没在水中的晶体基底（如高度取向热解石墨（HOPG））上的准二维微米级区域。它们只有几纳米厚，据推测是由溶解在固 - 水界面的空气积聚形成的。然而，自首次观察到微煎饼以来，其确切的化学性质和基本物理性质一直存在争议，主要原因是缺乏合适的表征技术。在本研究中，通过使用峰值力定量纳米力学（PF - QNM）模式原子力显微镜（AFM）研究了HOPG与乙醇 - 水溶液界面处微煎饼的硬度。我们的测量结果表明，微煎饼比纳米气泡更硬，对于双层微煎饼而言，与基底接触的底层比顶层更硬。有趣的是，随着溶液中乙醇浓度的增加，微煎饼变得更小、更软，并且在乙醇临界浓度以上无法通过AFM检测到。但在溶液中乙醇浓度降低后它们又重新出现。显然，微煎饼的演变和硬度取决于溶液中的化学成分，这可能归因于微煎饼的机械性能与液相表面张力之间的相关性。基于微煎饼随乙醇浓度的“出现 - 消失”行为，我们发现微煎饼实际上能够耐受远高于文献报道的5%的乙醇浓度。这项工作的结果可能有助于揭示微煎饼的化学性质。
