Microscopic fundamentals of the macroscopic interface formation principles

为了预测流体运动，需要指定域边界处的条件，特别是当流体遇到固体表面时。一百多年来，连续介质力学教科书中的标准无滑移边界条件，即固体基底上的流体没有相对速度，描述了我们的许多日常经验。然而，最近的实验表明，没有强有力的论据来证明无滑移条件，实际上发生部分滑移。这种行为的原因原来是一个薄的液体（界面）层分离液相和固相。该界面层的性质定义了液体滑移现象和毛细作用的影响，即与表面张力相关的影响，以及一般而言，整体流动的边界条件的性质。界面层变得重要时，系统的长度尺度收缩到微米级和物理行为在很大程度上是由高的表面体积比，这是微流体流动条件的情况下的影响。因此，人们已经理解，为了制定正确的边界条件，在微观尺度上的流体流动，有必要严格地描述界面层中的表面相的动态，表面相流与整体流的相互关系和界面形成的过程。总而言之，通常需要描述具有成形界面的毛细流动，具有成形界面的毛细流动是许多自然过程和技术应用的核心，从涂层设备、聚合物薄膜和新兴技术中使用的新工具，如微流体和纳米流体，到流体动力学的生物和医学应用。然而，我们对界面形成的基本原理的理解基本上是不完整的。这些界面层的定量描述的主要困难源于这样一个事实，即它们是具有本质上介观结构的物理系统，具有涉及宏观和分子尺度以及不同性质的力的强各向异性。基于分子动力学模拟的微观方法和基于不可逆热力学方法的宏观界面形成理论。直到最近，这两种方法已经变得足够成熟和强大，可以严格地处理表面相和界面形成的动力学方面，并且从第一微观原理进行这种基础研究的时机已经成熟。我们将通过微观分子动力学研究来寻找宏观理论的基础。这将允许测试和改进现有的宏观理论，并开发新的方法。这一建议的结果可以被广泛利用在不同的研究社区，定量模拟毛细流动形成接口。

To predict fluid motion, one needs to specify conditions at the domain boundaries, in particularly when a fluid meets a solid surface. For more than one hundred years, the standard no-slip boundary condition from the textbooks on continuum mechanics, which states that a fluid at a solid substrate has no relative velocity to it, has described much of our everyday experience. However, recent experiments have demonstrated that there is no strong argument to justify the no-slip condition and actually partial slip occurs. The reason for such behaviour turned out to be a thin liquid (interfacial) layer separating liquid and solid phases. The properties of this interfacial layer define the phenomenon of liquid slip and the effects of capillarity, that is, the effects related to the surface tension, and, in general, the nature of boundary conditions for the bulk flow. The interfacial layer becomes important when the system length scale shrinks to the microscale and the physical behaviour is largely influenced by high surface to volume ratio, which is the case for microfluidic flow conditions. So, it has been understood that in order to formulate correct boundary conditions for fluid flows at the microscale, it is necessary to describe rigorously the dynamics of the surface phase in the interfacial layer, interrelations of the surface phase flow with the bulk flow and the processes of interface formation. To summarise, generally one needs to describe capillary flows with forming interfaces.The capillary flows with forming interfaces are at the heart of numerous natural processes and technological applications ranging from coating devices, polymer films and new tools used in emerging technologies, such as micro and nano-fluidics, to biological and medical applications of fluid dynamics. Yet our understanding of fundamental principles of the interface formation is essentially incomplete. The main difficulty in quantitative description of these interfacial layers stems from the fact that they are physical systems with essentially mesoscopic structure which has strong anisotropy involving both macroscopic and molecular scales as well as forces of differing nature.The objective of this interdisciplinary project is to study fundamentals of the interface formation by bridging two major developments in interfacial science, the microscopic approach based on molecular dynamics simulations and the macroscopic theory of interface formation based on the methods of irreversible thermodynamics. Only recently the two approaches have become mature and powerful enough to rigorously address dynamical aspects of the surface phase and formation of interfaces and the time is ripe to perform such fundamental studies from the first microscopic principles.We are going to search for the foundation of macroscopic theories by means of microscopic molecular dynamics studies. This will allow to test and to improve the existing macroscopic theories and to develop new methodologies. The outcome of this proposal can be widely exploited in different research communities to quantitatively model capillary flows with forming interfaces.