Biological nanopores such as aquaporins combine the opposing functions of high water permeation and total ion exclusion in part by the virtue of their hourglass shape. Here, we perform molecular dynamics simulations to examine water and ion conduction through hourglass shaped nanopores created from carbon nanotubes (CNTs) of chirality (6,6), (8,8), and (10,10) in combination with carbon nanocones of half cone angles 41.8°, 30.0°, 19.45°, 9.6° and 0.0°. We observe large variations in flow through the nanopores with change in half cone angles and tube diameters. By computing the pore-water interactions we find a correlated change between the flux and the density profiles of water inside the nanopores. Further, from the orientation, and the hydrogen bonding characteristics of water, we uncover some unexplored facets of flow through hourglass shaped nanopores. The results are insightful for devising novel separation membranes based on nanopores that mimic the shape of biological nanochannels.

The intriguing mass transport properties of carbon nanotubes (CNTs) have received widespread attention, especially the rapid transport of water through CNTs due to their atomically smooth wall interiors. Extensive research has been dedicated to the comprehension of various aspects of water flow in contact with CNTs, the most prominent ones being the studies on slip and flow rates. Experimental and computational studies have confirmed an enhanced water flow rate through this graphitic nanoconfinement. However, a quantitative agreement has not yet been attained. These disparities coupled with incomplete knowledge of the mechanisms of water transport at nanoscale regimes are hindering the possibilities to integrate CNTs in numerous nanofluidic applications. In the present review, we focus on the slip and flow rates of water through CNTs and the factors influencing them. We discuss the key sources of discrepancies in water flow rate and suggest directions for future study.

Nanoporous carbon materials are extensively studied for various separation applications. Among them, water desalination by means of Reverse Osmosis (RO) stands out due to it's large socio-economic relevance. Many studies are carried out in this area both computationally and experimentally. In computational studies the water simulated using different water models are prone to produce inconsistent results. In this study water desalination through hydrogen functionalized graphene nanopore is studied using different water models (SPC, SPC/E, TIP3P, TIP4P/2005). Up to 81% difference was observed in the flux estimates among the models. The water permeation rate was found to be closely related to the bulk transport properties of the simulated water.

We introduce an analytical method to predict the slip length (L _s) in cylindrical nanopores using equilibrium molecular dynamics (EMD) simulations, following the approach proposed by Sokhan and Quirke for planar channels [39]. Using this approach, we determined the slip length of water in carbon nanotubes (CNTs) of various diameters. The slip length predicted from our method shows excellent agreement with the results obtained from nonequilibrium molecular dynamics (NEMD) simulations. The data show a monotonically decreasing slip length with an increasing nanotube diameter. The proposed EMD method can be used to precisely estimate slip length in high slip cylindrical systems, whereas, L _s calculated from NEMD is highly sensitive to the velocity profile and may cause large statistical errors due to large velocity slip at the channel surface. We also demonstrated the validity of the EMD method in a BNNT-water system, where the slip length is very small compared to that in a CNT pore of similar diameter. The developed method enables us to calculate the interfacial friction coefficient directly from EMD simulations, while friction can be estimated using NEMD by performing simulations at various external driving forces, thereby increasing the overall computational time. The EMD analysis revealed a curvature dependence in the friction coefficient, which induces the slip length dependency on the tube diameter. Conversely, in flat graphene nanopores, both L _s and friction coefficient show no strong dependency on the channel width.

We investigate thermally driven water droplet transport on graphene and hexagonal boron nitride (h-BN) surfaces using molecular dynamics simulations. The two surfaces considered here have different wettabilities with a significant difference in the mode of droplet transport. The water droplet travels along a straighter path on the h-BN sheet than on graphene. The h-BN surface produced a higher driving force on the droplet than the graphene surface. The water droplet is found to move faster on h-BN surface compared to graphene surface. The instantaneous contact angle was monitored as a measure of droplet deformation during thermal transport. The characteristics of the droplet motion on both surfaces is determined through the moment scaling spectrum. The water droplet on h-BN surface showed the attributes of the super-diffusive process, whereas it was sub-diffusive on the graphene surface.

Molecular dynamics simulations are widely employed to analyze water and ion permeation through nanoporous membranes for reverse osmosis applications. In such simulations, water models play an important role in accurately reproducing the properties of water. We investigated the water and ion transport across a hydroxyl (OH) functionalized graphene nanopore using six water models: SPC, SPC/E, SPC/Fw, TIP3P, TIP4P, and TIP4P/2005. The water flux thus obtained varied up to 84% between the models. The water and ion flux showed a correlation with the bulk transport properties of the models such as the diffusion coefficient and shear viscosity. We found that the hydrogen-bond lifetime, resulting from the partial charges of the model, influenced the flux. Our results are useful in the selection of a water model for computer simulations of desalination using nanomembranes. Our findings also suggest that lowering the hydrogen-bond lifetime and enhancing the rate of diffusion of water would lead to enhanced water/ion flux.

Although the importance of temperature control in nonequilibrium molecular dynamics simulations is widely accepted, the consequences of the thermostatting approach in the case of strongly confined fluids are underappreciated. We show the strong influence of the thermostatting method on the water transport in carbon nanotubes (CNTs) by considering simulations in which the system temperature is controlled via the walls or via the fluid. Streaming velocities and mass flow rates are found to depend on the tube flexibility and on the thermostatting algorithm, with flow rates up to 20% larger when the walls are flexible. The larger flow rates in flexible CNTs are explained by a lower friction coefficient between water and the wall. Despite the lower friction, a larger solid-fluid interaction energy is found for flexible CNTs than for rigid ones. Furthermore, a comparison of thermostat schemes has shown that the Berendsen and Nosé-Hoover thermostats result in very similar transport rates, while lower flow rates are found under the influence of the Langevin thermostat. These findings illustrate the significant influence of the thermostatting methods on the simulated confined fluid transport.

Thermal-gradient induced transport of ionic liquid (IL) and water droplets through a carbon nanotube (CNT) is investigated in this study using molecular dynamics simulations. Energetic analysis indicates that IL transport through a CNT is driven primarily by the fluid-solid interaction, while fluid-fluid interactions dominate in water-CNT systems. Droplet diffusion analysis via the moment scaling spectrum reveals sub-diffusive motion of the IL droplet, in contrast to the self-diffusive motion of the water droplet. The Soret coefficient and energetic analysis of the systems suggest that the CNT shows more affinity for interaction with IL than with the water droplet. Thermophoretic transport of IL is shown to be feasible, which can create new opportunities in nanofluidic applications.