Friction in liquids arises from conservative forces between molecules and atoms. Although the hydrodynamics at the nanoscale is subject of intense research and despite the enormous interest in the non-Markovian dynamics of single molecules and solutes, the onset of friction from the atomistic scale so far could not be demonstrated. Here, we fill this gap based on frequency-resolved friction data from high-precision simulations of three prototypical liquids, including water. Combining with theory, we show that friction in liquids emerges abruptly at a characteristic frequency, beyond which viscous liquids appear as non-dissipative, elastic solids. Concomitantly, the molecules experience Brownian forces that display persistent correlations. A critical test of the generalised Stokes–Einstein relation, mapping the friction of single molecules to the visco-elastic response of the macroscopic sample, disproves the relation for Newtonian fluids, but substantiates it exemplarily for water and a moderately supercooled liquid. The employed approach is suitable to yield insights into vitrification mechanisms and the intriguing mechanical properties of soft materials.

液体中的摩擦是由分子和原子之间的保守力引起的。尽管纳米级的流体动力学是一项深入的研究，尽管人们对单个分子和溶质的非马尔可夫动力学产生了极大的兴趣，但到目前为止，尚无法证明原子级的摩擦起因。在此，我们基于频率解析的摩擦数据填补了这一空白，该摩擦数据来自对三种原型液体（包括水）的高精度仿真。结合理论，我们发现液体中的摩擦力以一个特征频率突然出现，超过该频率时，粘性液体表现为非耗散的弹性固体。伴随地，分子经历布朗力，该布朗力显示出持久的相关性。对广义斯托克斯-爱因斯坦关系的严格检验，将单个分子的摩擦映射到宏观样品的粘弹性响应，可以证明与牛顿流体的关系，但对于水和中度过冷的液体，则可以示例性地证实。所采用的方法适合于深入了解玻璃化机理和柔软材料的有趣机械性能。