二维非均匀受限流体的动力学行为及其调控的分子模拟研究

The dynamical behaviors including transport properties and structural changes of nanoconfined fluids involve a number of complex mechanical, physical and chemical processes, and thus are a heat topic in many fields including Micro-/Nano- Mechanics, and hold the promise for wide applications in water treatment, new battery energy and the design of microelectromechanical/nanoelectromechanical systems (MEMS/NEMS). For instance, the lubricating film sandwiched in a friction pair presents in a typical two-dimensional confined state. Currently, theoretical and computational studies on the two-dimensional confined fluids are focusing on the model constructed with two parallel plates (i.e., homogeneous nanoconfinement). However, the tip end used in typical experimental systems of nano-tribology is not absolutely flat, such that the fluid between the tip and substrate presents in an inhomogeneously confined state. Distinct dynamical behaviors of inhomogeneously confined fluids will be presented for different structure, size or material and surface profile of the confined spaces. This project will develop the modeling and simulation methods for studying the inhomogeneously confined fluid, and then systematically investigate its dynamical behaviors including structure and phase transition as well as their regulation. Furthermore, we will construct a typical system for nano-tribology research containing inhomogeneously confined fluids between tip and substrate, in order to reveal the mechanism linking the friction and the dynamics of confined fluids to provide scientific basis for developing high-efficient lubrication technology for MEMS/NEMS.

纳米受限流体的输运和构象变化等动力学行为涉及复杂的力学、物理和化学过程，是微纳米力学等领域研究的前沿热点问题，在水处理、新能源及微纳机电系统等领域具有广阔的应用前景。例如，微纳机电系统中一对摩擦副之间的润滑膜就处于典型的二维受限状态下。目前，国际国内对于这些二维受限流体的理论研究一般采用平行板模型，即均匀受限。然而，实际的纳米摩擦实验系统中针尖的端部并非是完全平整的，因而受限在针尖和基底之间的流体处于非均匀受限状态下。非均匀受限空间的结构、尺寸、材质及表面形貌的不同会使受限流体呈现截然不同的动力学行为。本项目将发展用于研究非均匀受限流体的分子模型建模技术和计算方法，系统地开展二维非均匀受限流体的结构、相变等动力学特性及其调控机制的研究。进而，构建有二维非均匀受限流体参与润滑的纳米摩擦学体系，揭示其润滑性能与受限流体动力学特性之间的关联机制，为发展微纳机电系统的高效润滑技术提供科学基础。

本项目组通过理论建模和计算分析等手段系统研究了二维非均匀受限空间内流体的复杂动力学行为。发现单层、双层和三层液态水相在非均匀受限空间内的共存；当水的密度增加时，不同层数的受限液态水会依次转变为“冰纳米带”结构。这一液态至固态相变过程也可通过降温等方式来实现。此外，认识了针尖的尺寸、曲率半径，基底材料和表面形貌等特性对非均匀受限流体动力学行为的影响规律。在此过程中，掌握了非均匀受限体系建模和动力学行为分析技术；认识了非均匀受限流体的反常动力学特性及其微观机理；预测了有非均匀受限流体参与的纳米摩擦系统的摩擦特性。此外，我们还系统研究了基底表面、纳米孔等受限环境内流体和分子的异常动力学行为，发现受限空间的几何结构、表面特性对受限物质的行为有显著影响。本项目所取得的科学发现不仅扩展了受限流体的相图，也在纳米流体系统和多孔材料等领域有着重要的应用前景。在本项目的支持下，已在ACS Nano, Nano Letters等领域著名刊物上发表SCI论文5篇。