Jang Joonkyung, Yang Mino, Schatz George
Department of Nanomaterials Engineering, Pusan National University, Miryang 627-706, Republic of Korea.
J Chem Phys. 2007 May 7;126(17):174705. doi: 10.1063/1.2734548.
Water condenses between an atomic force microscope (AFM) tip and a surface to form a nanoscale bridge that produces a significant adhesion force on the tip. As humidity increases, the water bridge always becomes wider but the adhesion force sometimes decreases. The authors show that the humidity dependence of the adhesion force is intimately related to the structural properties of the underlying water bridge. A wide bridge whose width does not vary much with tip-surface distance can increase its volume as distance is increased. In this case, the adhesion force decreases as humidity rises. Narrow bridges whose width decreases rapidly with increasing tip-surface distance give the opposite result. This connection between humidity dependence of the adhesion force and the structural susceptibility of the water bridge is illustrated by performing Monte Carlo simulations for AFM tips with various hydrophilicities.
水在原子力显微镜（AFM）探针与表面之间凝结形成纳米级桥，该桥会在探针上产生显著的粘附力。随着湿度增加，水桥总是会变宽，但粘附力有时会降低。作者表明，粘附力对湿度的依赖性与底层水桥的结构特性密切相关。宽度随探针 - 表面距离变化不大的宽桥，随着距离增加其体积会增大。在这种情况下，随着湿度升高，粘附力会降低。宽度随探针 - 表面距离增加而迅速减小的窄桥则会产生相反的结果。通过对具有各种亲水性的AFM探针进行蒙特卡罗模拟，说明了粘附力对湿度的依赖性与水桥结构敏感性之间的这种联系。
