Li F, Korotkin I A, Karabasov S A
The School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, E1 4NS London, United Kingdom.
Mathematical Sciences, University of Southampton, University Road, SO17 1BJ Southampton, United Kingdom.
Langmuir. 2020 May 26;36(20):5633-5646. doi: 10.1021/acs.langmuir.0c01049. Epub 2020 May 15.
Water confined by hydrophilic materials shows unique transport properties compared to bulk water, thereby offering new opportunities for the development of nanofluidic devices. Recent experimental and numerical studies showed that nanoconfined water undergoes liquid- to solid-phase-like transitions depending on the degree of confinement. In the case of water confined by graphene layers, the van der Waals forces are known to deform the graphene layers, whose bending leads to further nonuniform confinement effects. Despite the extensive studies of nanoconfined water under equilibrium conditions, the interplay between the confinement and rheological water properties, such as viscosity, slip length, and normal stress differences under shear flow conditions, is poorly understood. The current investigation uses a validated all-atom nonequilibrium molecular dynamics model to simultaneously analyze the continuum transport and atomistic structural properties of water in a slit between two moving graphene walls under Couette flow conditions. A range of different slit widths and velocity strain rates are considered. It is shown that under subnanometer confinement, water loses the rotational symmetry of a Newtonian fluid. Under such conditions, water transforms into ice, where the atomistic structure is completely insensitive to the applied shear force and behaves like a frozen slab sliding between the graphene walls. This leads to the shear viscosity increase, although it is not as dramatic as the normal force increase that contributes to the increased friction force reported in previous experimental studies. On the other end of the spectrum, for flows at large velocity strain rates in moderate to large slits between the graphene walls, water is in the liquid state and reveals shear thinning behavior. In this case, water exhibits a constant slip length on the wall, which is typical of liquids in the vicinity of hydrophobic surfaces.
与大块水相比，被亲水性材料限制的水表现出独特的传输特性，从而为纳米流体装置的发展提供了新机遇。最近的实验和数值研究表明，纳米受限水会根据受限程度发生类似液相到固相的转变。在被石墨烯层限制的水的情况下，已知范德华力会使石墨烯层变形，其弯曲会导致进一步的非均匀受限效应。尽管对平衡条件下的纳米受限水进行了广泛研究，但在剪切流条件下，受限与水的流变特性（如粘度、滑移长度和法向应力差）之间的相互作用仍知之甚少。当前的研究使用经过验证的全原子非平衡分子动力学模型，在库埃特流条件下同时分析两个移动的石墨烯壁之间狭缝中水的连续介质传输和原子结构特性。考虑了一系列不同的狭缝宽度和速度应变率。结果表明，在亚纳米限域下，水失去了牛顿流体的旋转对称性。在这种条件下，水转变为冰，其原子结构对施加的剪切力完全不敏感，表现得像在石墨烯壁之间滑动的冻结平板。这导致剪切粘度增加，尽管不如法向力增加那么显著，而法向力增加导致了先前实验研究中报道的摩擦力增加。在另一个极端情况下，对于石墨烯壁之间中等至大狭缝中高速应变率的流动，水处于液态并表现出剪切变稀行为。在这种情况下，水在壁上表现出恒定的滑移长度，这是疏水表面附近液体的典型特征。
