Division of Molecular and Materials Simulation, Key Laboratory for Nanomaterials, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China.
J Chem Phys. 2009 Nov 14;131(18):184702. doi: 10.1063/1.3257624.
Atomic force microscopy (AFM) can work in a variety of environment with different humidities. When the tip of AFM approaches a sample, the measured adhesion force would be significantly affected by the presence of nanometer-sized liquid bridge. The formation and rupture of liquid bridges can occur either through equilibrium or nonequilibrium process. In this work, the liquid bridges are assumed to be in thermodynamic equilibrium with the surrounding vapor medium. To study theoretically the stability of liquid bridge, a constraint is added into the lattice density functional theory to stabilize a series of bridges with different radii at a given tip-substrate distance. With the help of the constraint, we can identify not only stable and metastable states but also transition states for the formation and rupture of liquid bridges. Using this constrained method we calculate the energy barriers involved in the formation and rupture of the liquid bridges, respectively, and then discuss their stability as well as the origin of the hysteresis behavior observed with atomic force microscope measurements. On the whole, the calculated force-distance curves are found to be qualitatively in agreement with experimental observations. The energy barriers for the formation and rupture of liquid bridges are also analyzed as a function of tip-sample distance, humidity, and tip-fluid interaction.
原子力显微镜（AFM）可以在不同湿度的多种环境中工作。当 AFM 的探针接近样品时，测量到的粘附力会受到纳米级液桥的存在显著影响。液桥的形成和破裂可以通过平衡或非平衡过程发生。在这项工作中，假设液桥与周围的蒸汽介质处于热力学平衡状态。为了从理论上研究液桥的稳定性，在格子密度泛函理论中添加了一个约束条件，以在给定的探针-基底距离下稳定一系列具有不同半径的桥。借助于约束条件，我们不仅可以识别稳定和亚稳定状态，还可以识别液桥形成和破裂的过渡状态。使用这种约束方法，我们分别计算了液桥形成和破裂所涉及的能量势垒，然后讨论了它们的稳定性以及原子力显微镜测量中观察到的滞后行为的起源。总的来说，计算出的力-距离曲线与实验观察结果定性一致。还分析了液桥形成和破裂的能量势垒作为探针-样品距离、湿度和探针-流体相互作用的函数。
