Farshchi-Tabrizi Mahdi, Kappl Michael, Cheng Yajun, Gutmann Jochen, Butt Hans-Jürgen
Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
Langmuir. 2006 Feb 28;22(5):2171-84. doi: 10.1021/la052760z.
In this study we measured the adhesion forces between atomic force microscope (AFM) tips or particles attached to AFM cantilevers and different solid samples. Smooth and homogeneous surfaces such as mica, silicon wafers, or highly oriented pyrolytic graphite, and more rough and heterogeneous surfaces such as iron particles or patterns of TiO2 nanoparticles on silicon were used. In the first part, we addressed the well-known issue that AFM adhesion experiments show wide distributions of adhesion forces rather than a single value. Our experiments show that variations in adhesion forces comprise fast (i.e., from one force curve to the next) random fluctuations and slower fluctuations, which occur over tens or hundreds of consecutive measurements. Slow fluctuations are not likely to be the result of variations in external factors such as lateral position, temperature, humidity, and so forth because those were kept constant. Even if two solid bodies are brought into contact under precisely the same conditions (same place, load, direction, etc.) the result of such a measurement will often not be the same as that of the previous contact. The measurement itself will induce structural changes in the contact region, which can change the value for the next adhesion force measurement. In the second part, we studied the influence of humidity on the adhesion of nanocontacts. Humidity was adjusted relatively fast to minimize tip wear during one experiment. For hydrophobic surfaces, no signification change in adhesion force with humidity was observed. Adhesion force versus humidity curves recorded with hydrophilic surfaces either showed a maximum or continuously increased. We demonstrate that the results can be interpreted with simple continuum theory of the meniscus force. The meniscus force is calculated based on a model that includes surface roughness and takes into account different AFM tip (or particle) shapes by a two-sphere model. Experimental and theoretical results show that the precise contact geometry has a critical influence on the humidity dependence of the adhesion force. Changes in tip geometry on the sub-10-nm length scale can completely change adhesion force versus humidity curves. Our model can also explain the differences between earlier AFM studies, where different dependencies of the adhesion force on humidity were observed.
在本研究中，我们测量了附着在原子力显微镜（AFM）悬臂上的AFM针尖或颗粒与不同固体样品之间的粘附力。使用了光滑且均匀的表面，如云母、硅片或高度取向的热解石墨，以及更粗糙和不均匀的表面，如铁颗粒或硅上的二氧化钛纳米颗粒图案。在第一部分，我们探讨了一个众所周知的问题，即AFM粘附实验显示出粘附力的广泛分布而非单一值。我们的实验表明，粘附力的变化包括快速（即从一条力曲线到下一条）随机波动和较慢的波动，后者发生在数十次或数百次连续测量过程中。缓慢波动不太可能是诸如横向位置、温度、湿度等外部因素变化的结果，因为这些因素保持恒定。即使两个固体在完全相同的条件下（相同位置、载荷、方向等）接触，这样一次测量的结果往往也与前一次接触不同。测量本身会在接触区域引起结构变化，这会改变下一次粘附力测量的值。在第二部分，我们研究了湿度对纳米接触粘附力的影响。在一次实验过程中，相对快速地调节湿度以尽量减少针尖磨损。对于疏水表面，未观察到粘附力随湿度有明显变化。在亲水表面记录的粘附力与湿度曲线要么显示出最大值，要么持续增加。我们证明，结果可以用简单的弯月面力连续介质理论来解释。弯月面力是基于一个包含表面粗糙度的模型计算得出的，并通过双球模型考虑了不同的AFM针尖（或颗粒）形状。实验和理论结果表明，精确的接触几何形状对粘附力的湿度依赖性有至关重要的影响。亚10纳米长度尺度上针尖几何形状的变化会完全改变粘附力与湿度曲线。我们的模型还可以解释早期AFM研究之间的差异，在这些研究中观察到了粘附力对湿度的不同依赖性。
