van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.
van der Waals-Zeeman Institute, Institute of Physics, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.
ACS Appl Mater Interfaces. 2023 Mar 8;15(9):12603-12608. doi: 10.1021/acsami.3c00789. Epub 2023 Feb 24.
When two solid objects slide over each other, friction results from the interactions between the asperities of the (invariably rough) surfaces. Lubrication happens when viscous lubricants separate the two surfaces and carry the load such that solid-on-solid contacts are avoided. Yet, even small amounts of low-viscosity lubricants can still significantly lower friction through a process called boundary lubrication. Understanding the origin of the boundary lubricating effect is hampered by challenges in measuring the interfacial properties of lubricants directly between the two surfaces. Here, we use rigidochromic fluorescent probe molecules to measure precisely what happens on a molecular scale during vapor-phase boundary lubrication of a polymer bead-on-glass interface. The probe molecules have a longer fluorescence lifetime in a confined environment, which allows one to measure the area of real contact between rough surfaces and infer the shear stress at the lubricated interfaces. The latter is shown to be proportional to the inverse of the local interfacial free volume determined using the measured fluorescence lifetime. The free volume can then be used in an Eyring-type model as the stress activation volume, allowing to collapse the data of stress as a function of sliding velocity and partial pressure of the vapor phase lubricant. This shows directly that as more boundary lubricant is applied, larger clusters of lubricant molecules become involved in the shear process thereby lowering the friction.
当两个固体物体相互滑动时，摩擦力是由（通常粗糙的）表面的凹凸部分之间的相互作用产生的。当粘性润滑剂将两个表面隔开并承载负载，从而避免固体与固体接触时，就会发生润滑。然而，即使是少量低粘度的润滑剂，通过一种称为边界润滑的过程，仍能显著降低摩擦力。由于难以直接在两个表面之间测量润滑剂的界面特性，因此理解边界润滑效应的起源受到阻碍。在这里，我们使用刚性变色荧光探针分子来精确测量聚合物球在玻璃上的界面在气相边界润滑过程中在分子尺度上发生的情况。探针分子在受限环境中的荧光寿命较长，这允许测量粗糙表面之间的实际接触面积，并推断润滑界面处的剪切应力。后者被证明与使用测量的荧光寿命确定的局部界面自由体积的倒数成正比。然后可以将自由体积用作 Eyring 型模型中的应力激活体积，从而可以将作为滑动速度和气相润滑剂分压函数的应力数据折叠起来。这直接表明，随着更多边界润滑剂的应用，更多的润滑剂分子簇参与到剪切过程中，从而降低了摩擦力。
