Fachbereich Physik, Freie Universität Berlin , Arnimallee 14, 14195 Berlin, Germany.
Nano Lett. 2017 Oct 11;17(10):5969-5976. doi: 10.1021/acs.nanolett.7b02000. Epub 2017 Sep 21.
The viscous properties of nanoscopically confined water are important when hydrated surfaces in close contact are sheared against each other. Numerous experiments have probed the friction between atomically flat hydrated surfaces in the subnanometer separation regime and suggested an increased water viscosity, but the value of the effective viscosity of ultraconfined water, the mechanism of hydration layer friction, and the crossover to the dry friction limit are unclear. We study the shear friction between polar surfaces by extensive nonequilibrium molecular dynamics simulations in the linear-response regime at low shearing velocity, which is the relevant regime for typical biological applications. With decreasing water film thickness we find three consecutive friction regimes: For thick films friction is governed by bulk water viscosity. At separations of about a nanometer the highly viscous interfacial water layers dominate and increase the surface friction, while at the transition to the dry friction limit interfacial slip sets in. Based on our simulation results, we construct a confinement-dependent friction model which accounts for the additive friction contributions from bulklike water, interfacial water layers, and interfacial slip and which is valid for arbitrary water film thickness.
当紧密接触的水合表面相互剪切时，纳米受限水中的粘性特性非常重要。许多实验研究了亚纳米分离范围内原子级平坦水合表面之间的摩擦，并表明水的粘度增加，但超受限水的有效粘度值、水化层摩擦的机制以及与干摩擦极限的转变尚不清楚。我们通过在低剪切速度的线性响应区进行广泛的非平衡分子动力学模拟来研究极性表面之间的剪切摩擦，这是典型生物应用的相关区域。随着水膜厚度的减小，我们发现了三个连续的摩擦区：对于较厚的膜，摩擦受体相粘度的控制。在大约一纳米的分离时，高粘性的界面水层占主导地位，增加了表面摩擦，而在向干摩擦极限的转变中，界面滑动开始出现。基于我们的模拟结果，我们构建了一个依赖于约束的摩擦模型，该模型考虑了来自类体水、界面水层和界面滑动的附加摩擦贡献，并且适用于任意水膜厚度。
