Colloidal Monolayers in Periodic Laser Fields

软物质系统在日常生活中经常遇到。许多清洁和化妆品、胶水、尿布、食品以及天然和生物材料都被认为是软物质。软物质的主要特征是它对外部扰动的敏感性，如机械力、电力或磁力。因此，软物质系统是软的，正如它们的名字所表明的那样。在这个项目中，我们研究胶体系统对外部势的响应。胶体系统是软物质的一部分，由分散在介质中的大小在几纳米到几微米之间的颗粒组成。我们使用排列在一层中的球形粒子，因此表示一个准二维系统。它们暴露在正弦光图案中，对粒子施加周期性电势，其中电势的波长约为粒子大小的数量级。将胶体粒子暴露在正弦电势下似乎是一种非常基本的情况。然而，令人惊讶的是，描述这种情况的理论模型很少。即使是非常基本的问题，比如这样的势中的粒子排列和动力学，几乎都没有被触及。为了取得进展，我们将实验、模拟和理论相结合。建立了理论模型，并将其预测结果与模拟和实验结果进行了比较。此外，实验还被用来指导和启发理论描述的发展。光学显微镜将被用来跟踪粒子。这将产生系统的和定量的信息，精确到单个粒子的水平，从而使我们能够确定任何从理论上得出的参数。将应用不同的理论方法，包括密度泛函理论、积分方程式理论和模式耦合理论。我们的目标是确定暴露在正弦电势下的粒子的排列和动力学。这种排列将根据密度分布和结构因素进行量化，而动力学将通过均方位移、扩散系数和中间散射函数来表征。此信息还将用于构建状态图中的玻璃化转变线。我们期待粒子排列和动力学的非单调行为，因为势的周期和幅度是不同的。通过研究暴露在调制势中的胶体粒子，我们将物质推向极端状态。这使得我们一方面可以探索材料的性质，对潜在的过程有更深的了解，另一方面可以制备和表征具有新性质的材料。

Soft matter systems are frequently encountered in everyday life. Many cleaning and cosmetic products, glue, nappies, food stuff as well as natural and biological materials are considered soft matter. The main characteristic of soft matter is its susceptibility to external perturbations, such as mechanical, electrical or magnetic forces. Thus, soft matter systems are soft, as suggested by their name.Within this project we investigate the response of a colloidal system to an external potential. Colloidal systems are part of soft matter and consist of particles with a size between a few nanometres and a few micrometres that are dispersed in a medium. We use spherical particles that are arranged in one layer and thus represent a quasi-two-dimensional system. They are exposed to a sinusoidal light pattern that imposes a periodic potential on the particles, where the wavelength of the potential is of the order of the particle size. Exposing colloidal particles to a sinusoidal potential appears as a very fundamental situation. Surprisingly, however, theoretical models describing this situation are rare. Even very basic questions, like the particle arrangement and dynamics in such a potential, have been virtually untouched. To make progress, we combine experiments, simulations and theory. A theoretical model will be developed and its predictions compared with simulation and experimental results. The experiments furthermore are used to guide and inspire the development of the theoretical description.Optical microscopy will be used to follow the particles. This will yield systematic and quantitative information down to the level of individual particles and hence allow us to determine any parameter that is derived theoretically. Different theoretical approaches will be applied, including density-functional theory, integral-equation theory and mode-coupling theory. We aim at a determination of the arrangement and dynamics of particles exposed to a sinusoidal potential. The arrangement will be quantified in terms of density profiles and structure factors, while the dynamics will be characterized by the mean-square displacement, diffusion coefficient and intermediate scattering function. This information will also be used to construct the glass-transition lines in the state diagram. We anticipate a non-monotonic behaviour of the particle arrangement and dynamics as the periodicity and amplitude of the potential is varied.By studying colloidal particles exposed to a modulated potential, we push matter to extreme states. This allows us on one hand to probe material properties and gain a deeper understanding of the underlying processes and on the other hand to prepare and characterize materials with new properties.