Friction of colloidal monolayers on periodic and quasiperiodic surfaces

摩擦是一个重要的过程，在很大的长度范围和几乎任何材料中都会遇到。根据具体情况，需要高摩擦系数或低摩擦系数，例如，增强衬垫的粘附性或减少发动机中运动部件之间的磨损。尽管历史悠久，但人们对摩擦的许多方面还没有很好地了解。在这个建议中，我们将研究摩擦和接触表面的结构特性之间的基本关系。例如，我们将研究具有周期和准周期顺序的表面，已知它们的摩擦系数表现相当不同。该研究将使用微米级颗粒的胶体模型系统进行，该系统通过周期性和准周期性衬底驱动，后者由光学干涉图案创建。由于光学梯度力的作用，这种模式起着基底电位的作用，其长度尺度、相互作用强度甚至几何形状都可以连续变化。这种方法的另一个优点是可以畅通无阻地进入摩擦过程发生的界面。这允许研究如何弹性和塑性变形或爬行运动发生与单粒子分辨率。根据两个物体是否在注册表（相称）或不相称（不相称），可能出现粘滑或连续滑动运动。由于后者与低摩擦系数密切相关，我们的研究不仅将为摩擦的微观机制提供有价值的见解，而且有助于理解超低摩擦系数（超润滑）的预期条件。

Friction is an important process encountered at a wide range of length scales and in almost any material. Depending on the specific situation, either strong or low friction coefficients are desirable, e.g. to enhance adherence of adlayers or to reduce wear between moving components in engines. Despite its long history, however, many aspects of friction are not well understood. In this proposal we will investigate the basic relationship between friction and the structural properties of the surfaces in contact. Exemplarily, we will study surfaces with periodic and quasiperiodic order which are known to behave rather differently regarding their friction coefficients. The studies will be performed with a colloidal model system of micron-sizes particles which is driven across periodic and quasiperiodic substrates, the latter created by optical interference patterns. Due to optical gradient forces, such patterns play the role of a substrate potential whose length scale, interaction strength and even geometry can be continuously varied. An additional advantage of this approach is the possibility to have unobstructed optical access to the interface where the friction process occurs. This allows to study how elastic and plastic deformations or creeping motion occurs with single particle resolution. Depending on whether the two bodies are in registry (commensurate) or not (incommensurate), either a stick-slip or a continuous sliding motion is expected. Since the latter is strongly related to low friction coefficients, our research will not only add valuable insights into the microscopic mechanisms of friction but also help to understand the conditions under which ultralow friction coefficients (superlubricity) can be expected.