Mechanics of Nanoscale Single Asperity Contacts in Friction Force Microscopy

许多新技术都依赖于纳米结构的器件和材料来提供新的特性和改进的性能。许多日常技术也依赖于纳米尺度的材料组织。头发和织物护发素的作用就是一个很好的例子，它们的性能依赖于控制护发素分子在复杂弯曲表面上的分布的能力（头发纤维直径只有100微米，纺织纤维可能更小），并且需要在非常小的长度尺度上均匀分布。在所有这些领域，迫切需要以纳米级分辨率获得分子在表面分布的信息。然而，很少有方法可以做到这一点。摩擦力显微镜（FFM）为这个问题提供了一种解决方案，它使用附着在柔性微悬臂上的尖锐尖端来测量表面摩擦力。除了提供一种绘制表面组成的方法外，FFM还提供了一种理想的模型系统，用于理解新型小型化技术（如微机电设备）中出现的滑动触点类型，在这些技术中，微小的滑动触点需要润滑，而传统的润滑剂则失效。使用FFM解决这些不同问题的主要困难是，我们仍然缺乏对支撑其运作机制的基本原则的充分理解。为了获得关于表面摩擦的定量信息，我们必须首先能够理解与尖端-样品相互作用相关的接触力学-微观物理相互作用决定了摩擦相互作用的强度。关于这个问题有很多争论。一些研究人员倾向于使用一个非常古老的物理定律——阿蒙顿定律，该定律简单地说明了摩擦力与垂直于样品表面的载荷成正比。其他人则建议适用更复杂的法律。申请人最近取得了重大进展，他们首次表明，不仅摩擦相互作用的强度，而且所应用的力学类型似乎也受到进行FFM实验的环境的强烈影响。本建议的目标是在这些初步调查结果的基础上，建立对FFM机制的广泛理解。这种冒险将为以可靠的实验数据为基础的技术提供解释框架。除了更好地理解基本原理外，我们还旨在将该技术应用于两类重要的材料：有机聚合物（聚苯乙烯和聚甲基丙烯酸甲酯），其中分子量决定了材料的许多机械性能；聚合物刷，这种新材料正吸引着人们的极大兴趣，因为它们具有控制界面相互作用（如粘附）的潜力。在这两种情况下，FFM都可以提供一种比目前可用的其他技术更快更容易的方法来探索分子结构和性质，并且它可以证明是研究各种问题的研究人员的有价值的工具。

Many new technologies are relying upon nanostructured devices and materials to deliver new properties and improved performance. Many everyday technologies also depend upon the organisation of materials at the nanometre scale. A good illustration of this is the action of hair and fabric conditioners, which rely for their performance on the ability to control the distribution of conditioner molecules at complex, curved surfaces (hair fibres are only 100 micrometres in diameter, and textile fibres may be much smaller) with uniform distribution being required on very small length scales. In all of these areas, there is an urgent need for information about the distribution of molecules at the surface with a resolution of nanometres. However, there are very few ways of doing this. Friction force microscopy (FFM), which uses a sharp tip attached to a flexible microscopic cantilever to measure surface friction, provides one solution to this problem. In addition to providing a means of mapping surface composition, FFM also provides an ideal model system for understanding the types of sliding contacts that occur in new miniaturised technologies such as microelectromechanical devices, where tiny sliding contacts require lubrication but where conventional lubricants fail.The principal difficulty with using FFM to solve these varied problems is that we still lack an adequate understanding of the fundamental principles that underpin its mechanism of operation. In order to obtain quantitative information about surface friction, we must first be able to understand the contact mechanics associated with the tip-sample interaction - the microscopic physical interactions that determine the strength of the frictional interaction. There has been a great deal of debate about this. Some researchers have favoured the use of a very old physical law, Amontons' law, which states simply that the friction force is proportional to the load applied perpendicular to the sample surface. Others have suggested that more complex laws apply. Recent significant progress was made by the applicants, who showed for the first time that not just the strength of the frictional interaction, but also the type of mechanics that applied seemed strongly influenced by the environment in which the FFM experiment was conducted. The objective of this proposal is to build on these preliminary findings, by building a broad understanding of the mechanics of FFM. Such a venture will provide an interpretational framework for the technique that is grounded in solid experimental data. In addition to developing a better understanding of fundamental principles, we also aim to apply the technique to two important classes of materials: organic polymers (polystyrene and polymethylmethacrylate), where the molecular weight determines many of the mechanical properties of the material; and polymer brushes, new materials that are attracting enormous interest because of the potential they offer for control of interfacial interactions such as adhesion. In both cases, FFM may provide a quicker and easier method for exploring molecular structure and properties than other techniques currently available, and it could prove a valuable tool to researchers working on a variety of problems.