Water confined by hydrophilic materials shows unique transport properties compared to bulk water thereby offering new opportunities for development of nano-fluidic devices. Recent experimental and numerical studies showed that nano-confined 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 non-uniform confinement effects. Despite the extensive studies of nano-confined water at 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 non-equilibrium molecular dynamics model to simultaneously analyse continuum transport and atomistic structure 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 the sub-nanometer confinement, water loses its rotational symmetry of a Newtonian fluid. In such conditions, water transforms into ice, where the atomistic structure is completely insensitive to the applied shear force and which behaves like a frozen slab sliding between the graphene walls. This leads to the shear viscosity increase, although 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 spectra, for flows at large velocity strain rates in moderate to large slits between the graphene walls, water is in the liquid state and reveals a 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.

Artificial water nanochannels (AWCs) have drawn great attention due to their potential use in water purification. Herein, we propose an AWC design, which is based on coordinatively functionalizing unsaturated metal sites found in metal−organic frameworks (MOFs) with one-dimensional nanochannels. As a computational demonstration, we consider two MOFs, namely, Ni-CPO-27 and Ni-CPO-54, and graft proline, imidazolecarboxylic acid, imidazolecarboxaldehyde, pyrazolecarboxylic acid, and pyrazole carbaldehyde molecules into the MOF nanochannels. To assess the strength of the molecule-metal binding, binding energies were calculated using density functional theory. The results indicate that the MOFs containing either proline or 2-imidazolecarboxylic acid form waterstable AWCs with binding energies twice that of the binding energy of water. To shed light on the water diffusion mechanism in the proline-Ni-CPO-27/54 and 2-imidazolecarboxylic-Ni-CPO-27/54 AWCs, molecular dynamics simulations were performed to calculate the mean-squared displacement of water molecules and nonbonded interaction energies between select pairs of atoms in water and coordinated molecules were analyzed. It was found that the fastest water diffusion occurs in proline-Ni-CPO-54 with a self-diffusion coefficient of 7.2 ± 0.5 × 10 −8 cm 2 /s. In comparison, the fastest water self-diffusion coefficient reported in a carbon nanotube-based AWC is 9 × 10 −6 cm 2 /s. Nonbonded interactions between specific atom pairs regulate water diffusion in the functionalized MOF nanochannels. In particular, the change in water mean-squared displacement with changing water loading correlates well with the nonbonded energies between the partially positively charged hydrogen atoms in water and the partially negatively charged oxygen and nitrogen atoms in the proline and 2imidazolecarboxylic acid molecules. The results presented herein indicate that water-stable MOFs could perform well as AWCs, thereby lending support to the further design and synthesis of MOF-based AWCs for water purification.

Structured water near surfaces is important in nonclassical crystallization, biomineralization, and restructuring of cellular membranes. In addition to equilibrium structures, studied by atomic force microscopy (AFM), high-speed AFM (H-S AFM) can now detect piconewton forces in microseconds. With increasing speeds and decreasing tip diameters, there is a danger that continuum water models will not hold, and molecular dynamic (MD) simulations would be needed for accurate predictions. MD simulations, however, can only evolve over tens of nanoseconds due to memory and computational efficiency/speed limitations, so new methods are needed to bridge the gap. Here, we report a hybrid, multiscale simulation method, which can bridge the size and time scale gaps to existing experiments. Structured water is studied between a moving silica AFM colloidal tip and a cleaved mica surface. The computational domain includes 1,472,766 atoms. To mimic the effect of long-range hydrodynamic forces occurring in water, when moving the AFM tip at speeds from 5 × 10 −7 to 30 m/s, a hybrid multiscale method with local atomistic resolution is used, which serves as an effective open-domain boundary condition. The multiscale simulation is thus equivalent to using a macroscopically large computational domain with equilibrium boundary conditions. Quantification of the drag force shows the breaking of continuum behavior. Nonmonotonic dependence on both the tip speed and distance from the surface implies breaking of the hydration layer around the moving tip at time scales smaller than water cluster formation and strong water compressibility effects at the highest speeds.

Confined liquid has attracted great attention due to its potential applications in nanofluidic devices. With the development of liquid-cell transmission electron microscopy (LC-TEM), investigating the behaviors of confined liquid can be realized in real time. However, the dynamics of the liquid layer in liquid cells have not been fully understood.Here, nanoparticles (NPs) adhered to the cell window membranes are used as reference objects to study the flow regime of the liquid layer, which causes cooperative motion of the membranes and the NPs. Two categories of motion behaviors are investigated. One is the contraction of NPs toward the interior viewing area which results from the spreading out of the liquid to the surrounding region, with the bending of the membranes increasing with the loss of liquid in the viewing area. The other motion behavior is the occasional movement of all the NPs in the same direction with the directional movement of the liquid layer. This work offers a new method to study the dynamics of liquids by LC-TEM, the discoveries of which are valuable for understanding the confined liquid dynamics.

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