受限液体摩擦特性的分子动力学模拟研究

Understanding friction plays a central role in technological applications and phenomena in diverse fields ranging from micromechanical devices to bioengineering and to earthquakes. In particular, given the continuing miniaturization of mechanical devices towards the nanoscale, improved understanding of friction and wear could help in reducing energy consumption, improving reliability and extending service life of the devices. Indeed, an important part of their design process typically consists of trying to minimize friction and to eliminate stick-slip dynamics. Therefore, a key question of friction research is to understand how the presence or absence of stick-slip depends on the microscopic or atomic scale properties of the system. In this project, we study the effect of atomic scale surface-lubricant interactions on nanoscale boundary-lubricated friction, by considering a thin water film confined by two graphene sheets in molecular dynamics simulations. The effect of wettability of the confining graphene surfaces on lubricated friction will be studied by the simulations to understand the underlying atomic scale processes and mechanisms responsible for the presence or absence of stick-slip. The classic laws stating that friction is proportional to the applied load, independent of the apparent contact area and sliding velocity have been questioned in systems with dimensions approaching the nanometer. New experimental results have shown that the friction force varies with the sliding velocity in a manner that depends on the wettability of the surface. Hydrophilic surfaces exhibit a friction that decreases with sliding velocity, however, hydrophobic surfaces have the behavior that is opposite to that of hydrophilic one. In this project, we will also study the sliding velocity dependence of friction of water confined by two surfaces with different wettabilities by molecular dynamics simulations to have a better understanding of this phenomenon at nanoscale.

摩擦和磨损在机械、生物工程、地震等领域扮演着至关重要的角色，尤其是机械设备的尺寸持续向微小型化发展，提升人们在微观尺度下对摩擦和磨损的认识可以有助于减少能量的损耗、提高机械设备的可靠性和延长机械的使用寿命。在摩擦学领域,一个重要的课题就是从微观尺度上去理解粘滞-滑动效应的成因。在本项目中，申请人将通过分子动力学模拟的手段来研究水受限于两块石墨烯薄膜之间的摩擦现象，从纳米尺度上去理解水对固体表面的浸润能力是如何对摩擦力、粘滞-滑动效应产生影响的。此外，一条关于摩擦力的古老的观点认为，摩擦力正比于物体所受到的压力，与接触面积和滑动速度无关，而这一规律在纳米尺度下并不适用。目前，最新的实验结果表明，在亲水表面下，摩擦力随着相对滑动速度的增大而减少，但是在疏水表面下，摩擦力却随着相对滑动速度增大而增大。在本项目里，申请人将通过分子动力学模拟的手段去进一步研究在纳米尺度上造成这一现象的原因。

在本项目中我们通过分子动力学模拟的方法研究了液晶大分子在受限剪切流场下的热力学与动力学特性。我们考虑的体系为粗粒化的链状液晶分子模型，受限在两块晶格平板内，上面的平板以一定的剪切速度运动。液晶分子的取向会被平板的晶体结构所影响，并且在不同温度和不同的相互作用能量下液晶分子会表现出不同的特性。我们还给出了在高温下液晶分子的滑动长度与相互作用能之间的依赖关系，与理论预测相符合。在强相互作用能量下，滑动长度与剪切速度为线性的增长关系，在弱相互作用下则没有这样的线性关系。随后，我们通过全原子分子动力学模拟的手段，研究了液晶层厚度与摩擦力间的关系，随着厚度的增加，固体表面的结构对摩擦力的影响逐渐减弱，我们观察到在厚度超过一定数值时，其粘滞系数和体相的液晶溶液一致。我们随后研究了液晶和己烷的混合物，发现改变液晶与己烷的浓度比可以改变液体的摩擦系数。