Post Matthias, Wolf Steffen, Stock Gerhard
Biomolecular Dynamics, Institute of Physics, Albert Ludwigs University, Freiburg 79104, Germany.
J Chem Theory Comput. 2022 May 10;18(5):2816-2825. doi: 10.1021/acs.jctc.2c00190. Epub 2022 Apr 20.
The friction coefficient of fluids may become a function of the velocity at increased external driving. This non-Newtonian behavior is of general theoretical interest and of great practical importance, for example, for the design of lubricants. Although the effect has been observed in large-scale atomistic simulations of bulk liquids, its theoretical formulation and microscopic origin are not well understood. Here, we use dissipation-corrected targeted molecular dynamics, which pulls apart two tagged liquid molecules in the presence of surrounding molecules, and analyze this nonequilibrium process via a generalized Langevin equation. The approach is based on a second-order cumulant expansion of Jarzynski's identity, which is shown to be valid for fluids and therefore allows for an exact computation of the friction profile as well of the underlying memory kernel. We show that velocity-dependent friction in fluids results from an intricate interplay of near-order structural effects and the non-Markovian behavior of the friction memory kernel. For complex fluids such as the model lubricant CH, the memory kernel exhibits a stretched-exponential long-time decay, which reflects the multitude of timescales of the system.
在增加外部驱动力的情况下，流体的摩擦系数可能会成为速度的函数。这种非牛顿行为具有普遍的理论意义和重大的实际重要性，例如对于润滑剂的设计。尽管在大块液体的大规模原子模拟中已经观察到了这种效应，但其理论表述和微观起源尚未得到很好的理解。在这里，我们使用耗散校正的靶向分子动力学方法，即在周围分子存在的情况下拉开两个标记的液体分子，并通过广义朗之万方程分析这个非平衡过程。该方法基于雅尔津斯基恒等式的二阶累积量展开，结果表明它对流体是有效的，因此可以精确计算摩擦轮廓以及潜在的记忆核。我们表明，流体中与速度相关的摩擦是由近程结构效应和摩擦记忆核的非马尔可夫行为的复杂相互作用引起的。对于像模型润滑剂CH这样的复杂流体，记忆核呈现出拉伸指数型的长时间衰减，这反映了系统的多个时间尺度。
