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.

中文翻译：

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纳米约束的水化摩擦：从大体积通过界面到干摩擦

当紧密接触的水合表面相互剪切时，纳米级受限水的粘性非常重要。许多实验已经探究了在亚纳米级分离状态下原子平坦的水合表面之间的摩擦，并提出了增加的水粘度，但是超载水的有效粘度，水化层摩擦的机理以及与干摩擦极限的交点的值是不清楚。我们通过在低剪切速度下线性响应范围内的广泛的非平衡分子动力学模拟研究极性表面之间的剪切摩擦，这是典型生物学应用的相关范围。随着水膜厚度的减小，我们发现了三个连续的摩擦状态：对于厚膜，摩擦力由总体水粘度决定。