Faculty of Physics, Al. I. Cuza University, blvd. Carol I, no. 11, 700506 Romania.
Langmuir. 2012 Feb 7;28(5):2558-66. doi: 10.1021/la202917d. Epub 2012 Jan 25.
Transport mechanisms involved in capillary condensation of water menisci in nanoscopic gaps between hydrophilic surfaces are investigated theoretically and experimentally by atomic force microscopy (AFM) measurements of capillary force. The measurements showed an instantaneous formation of a water meniscus by coalescence of the water layers adsorbed on the AFM tip and sample surfaces, followed by a time evolution of meniscus toward a stationary state corresponding to thermodynamic equilibrium. This dynamics of the water meniscus is indicated by time evolution of the meniscus force, which increases with the contact time toward its equilibrium value. Two water transport mechanisms competing in this meniscus dynamics are considered: (1) Knudsen diffusion and condensation of water molecules in the nanoscopic gap and (2) adsorption of water molecules on the surface region around the contact and flow of the surface water toward the meniscus. For the case of very hydrophilic surfaces, the dominant role of surface water transportation on the meniscus dynamics is supported by the results of the AFM measurements of capillary force of water menisci formed at sliding tip-sample contacts. These measurements revealed that fast movement of the contact impedes on the formation of menisci at thermodynamic equilibrium because the flow of the surface water is too slow to reach the moving meniscus.
通过原子力显微镜（AFM）测量毛细力，从理论和实验两方面研究了水弯月面在亲水表面纳米间隙中毛细凝结的输运机制。测量结果表明，通过吸附在 AFM 针尖和样品表面的水层的聚合并立即形成水弯月面，然后弯月面的形状随时间演变而达到对应于热力学平衡的稳定状态。水弯月面的这种动力学由弯月面力的时间演变表示，该力随接触时间的增加而向平衡值增加。在这种弯月面动力学中，考虑了两种相互竞争的水输运机制：（1）纳米间隙中水蒸气的克努森扩散和凝结；（2）表面附近接触区水分子的吸附以及表面水向弯月面的流动。对于非常亲水的表面，通过测量在滑动针尖-样品接触处形成的水弯月面的毛细力的 AFM，支持了表面水输运对弯月面动力学的主导作用。这些测量结果表明，接触的快速移动会阻碍平衡状态下弯月面的形成，因为表面水的流动速度太慢而无法到达移动的弯月面。
