Structure and dynamics of liquids in confinement

从理论的角度来看，强受限流体中的输运具有重要的意义，从用于胶体颗粒悬浮液的微流体装置，到微观尺度的回火，再到薄膜的摩擦和润滑等各种应用都具有重要的意义。当约束长度远远大于颗粒直径时，结构基本上保持不变，动力学可以用流体力学的复杂本构方程来描述，一旦约束区域与颗粒间距离相当，结构和动力学就会发生巨大变化。最突出的效应被称为分层，其中密度剖面显示粒子靠近壁面的积聚，随后由于粒子的紧密堆积而产生典型的振荡。此外，局部近程填料与围壁的相互作用显著影响对分布函数等两点相关性。在动力学方面，这种竞争预计会极大地影响输运系数，特别是在被称为玻璃化转变的结构停滞附近。该项目的目标是提供窄缝内致密流体的结构和动力学特性的表征。这个问题是从模拟、理论和实验的角度来解决的。感兴趣的量是适合于相干和自动力学的广义中间散射函数。我们的理论方法依赖于最近发展的约束问题的模式耦合理论，并使用相干中间散射函数作为原始变量。该理论的可靠数值实现是该项目的核心。然后，计算非平衡态图，得到表征玻璃态冻结结构的适当的广义非遍历性参数。更一般地说，该理论应提供中间散射函数的完全时间依赖性。通过对自动力学的扩展，可以预测均方位移和扩散系数。由于该理论需要静态特性作为输入，我们也打算阐明极受限流体的行为。理论上的努力将通过对狭缝或楔形几何结构中稍微多分散的硬球系统或混合物的模拟以及对限制中胶体悬浮液的一系列实验加以补充。主要目标是测量理论中引入的相关观测值，并在理论、模拟和实验之间进行仔细的比较。该项目的好处是首次对强约束下的动力学进行微观描述，并通过计算机和实验室实验进行了彻底的测试。

Transport in strongly confined fluid is of fundamental interest both from a theoretical point of view as well as for various applications ranging from microfluidic devices for suspensions of colloidal particles, tempering on the microscale, to friction and lubrication of thin films. While for confinement lengths much larger than the particle diameter, the structure remains essentially unperturbed and dynamics can be described by hydrodynamics with a possibly complicated constitutive equation, the structure and dynamics is drastically changed once the confining region becomes comparable to the interparticle distance. The most prominent effect is known as layering where the density profile displays an accumulation of particles close to a wall followed by typical oscillations due to close packing of particles. Furthermore two-point correlations such as the pair-distribution function are significantly affected by the interplay of the local short-range packing and the confining walls. For the dynamics this competition is expected to drastically influence transport coefficients, in particular, in the vicinity of structural arrest referred to as glass transition. The goal of this project is to provide a characterization of the structural and dynamical properties of dense fluids confined to narrow slits. The problem is addressed both from a simulational, a theoretical, as well as an experimental point of view. The quantities of interest are suitably adapted generalized intermediate scattering functions both for the coherent as well as the self dynamics. Our theoretical approach relies on a recently developed mode-coupling theory for the confinement problem and uses coherent intermediate scattering functions as primordial variables. A reliable numerical implementation of the theory is at the heart of the project. Then, a non-equilibrium-state diagram can be calculated and the suitably generalized nonergodicity parameter characterizing the frozen-in structure in the glassy state can be obtained. More generally, the theory should provide the complete time dependence of the intermediate scattering functions. Using extensions to the self dynamics the mean-square displacements and the diffusion coefficients can be predicted. Since the theory requires static properties as input we intend also to elucidate the behavior of an extremely confined fluid. The theoretical effort is to be complemented by a simulation for slightly polydisperse hard-sphere systems or mixtures in slit or wedge geometry as well a series of experiments on colloidal suspensions in confinement. The primary goal is to measure the relevant observables as introduced in the theory and make a careful comparison between theory, simulation, and experiments. The benefit of the project is to arrive at a first microscopic description for the dynamics in strong confinement that is thoroughly tested versus computer and laboratory experiments.