Modifying the Boundary Conditions of Lubricant Flow

这项工作将为连续介质流体动力学的“棍棒”边界条件提供一个关键的实验评估。棒状假设是目前摩擦学中流体流动计算的基石之一，但最近的理论和实验提出了一种令人兴奋的可能性，即表面可以工程地产生部分滑移的边界条件。该方法将包括在尚不存在系统的先前实验的情况下测试连续介质流体力学的经典雷诺方程：部分湿润表面(流体在表面上的有限接触角)和可变表面粗糙度(粗糙度将是地形、化学和润滑添加剂)。对于测量，改进的表面力装置将在足够大的间距(10-200 nm)下运行，以使相互作用的牛顿流体被认为表现为一个连续体。将执行膜间距的纳米级振荡调制，使得调制的幅度和频率独立变化，允许平均速度在宽范围内变化，而不会大幅改变膜厚度。首先，我们将直接测试最近的理论预测和分子动力学模拟，当低粘度流体无法完全润湿光滑的固体边界时，这些预测棒边界条件的崩溃。其次，我们将把这项工作扩展到“片状”表面--化学上和几何上粗糙的表面--以验证流体流动的“粘滞”边界条件源于粗糙度的理论预测。第三，我们将把这项工作扩展到“毛状”表面--带有表面附着聚合物链的表面。这就是由于聚合物添加剂的存在而在油润滑中普遍存在的情况。这是由于表面附着的生物大分子的存在而在生物溶解中盛行的情况。工程影响将是为润滑油流动过程中控制能量消耗的新方法指明方向。主要的应用将与其说是关于重型机器的高负载条件，不如说是关于新兴的纳米技术和微型机器的低负载条件。

0119626Granick This work will provide a critical experimental assessment of the"stick" boundary condition of continuum hydrodynamics. The stick assumption is presently one of the cornerstones of calculation regarding fluid flow in tribology yet recent theories and experiments raise the exciting possibility that surfaces can be engineering to produce the boundary condition of partial slip. The approach will consist of testing the classical Reynolds Equation of continuum hydrodynamics for cases where systematic prior experiments do not yet exist: partially-wetted surfaces (finite contact angle of the fluid on the surface) and variable surface roughness (the roughness will be topographical, chemical, and lubrication additives). For measurement, a modified surface forces apparatus will be operated at spacings large enough (10-200 nm) that intervening Newtonian fluids are believed to behave as a continuum. Nanometer-level oscillatory modulations of film spacing will be performed such that the amplitude and frequency of modulation are varied independently, allowing the mean velocity to vary over a wide range without large change of the film thickness. First, we will put to direct test recent theoretical predictions and molecular dynamics simulations that predict breakdown of stick boundary conditions when low-viscosity fluids fail to wet completely a smooth solid boundary. Secondly, we will extend this work to" patchy" surfaces - surfaces that are chemically and geometrically rough - to test theoretical predictions that" stick" boundary conditions of fluid flow stem from the presence of roughness. Third, we will extend this work to "hairy" surfaces - surfaces that bear surface-attached polymer chains. This is the situation that prevails in oil lubrication owing to the presence of polymer additives. It is the situation that prevails in biolubrication owing to the presence of surface-attached biomacromolecules. The engineering impact will be to point the way to new ways by which energy dissipation can be controlled during lubricant flow. The main applications will pertain not so much to the high-load conditions characteristic of heavy machines, as to the low-load conditions characteristic of emerging nanotechnologies and micromachines.***