Scale Effect on Simple Liquid Transport through a Nanoporous Graphene Membrane

Applied Electric Field Effects on Diffusivity and Electrical Double-Layer Thickness

This study utilizes molecular dynamics (MD) simulations and continuum frameworks to explore electroosmotic flow (EOF) in nanoconfined aqueous electrolytes, offering a promising alternative to conventional micro-/nanofluidic systems. Although osmotic behavior in these environments is deeply linked to local fluid properties and interfacial dynamics between the fluid and electrolyte solutions, achieving a complete molecular-level understanding has remained challenging. The findings establish a linear relationship between electric field strength and fluid velocity, uncovering two distinct transport regimes separated by a critical threshold, with a markedly intensified flow in the second regime. It is demonstrated that rising electric field strengths significantly enhance water diffusion coefficients, supported by a detailed analysis of fluid hydration structures, the potential of mean force (PMF), and local stress tensors. Due to the applied electric field strength, the motion of ions and water accelerates, leading to the redistribution of ions and intensification of electrostatic forces. This expands the thickness of the electric double layer (EDL) and amplifies fluid diffusivity, thereby enhancing nanoscale fluid activity. These insights enhance the molecular-level understanding of EOF and define the stability of flow regimes, providing valuable guidelines for advancing nanofluidic technologies, such as drug delivery systems and lab-on-a-chip devices.

Passive fractionating mechanism for oil spill using shear-wettability modulation

Oil spillage and organic solvent leakage have been a frequent occurrence in recent years, which pose a significant threat not only to the aquatic ecosystems but also result in substantial economic burdens. This has necessitated the search for materials capable of separating oil from water at enhanced efficiency with superior mechanical and thermal properties. In this study, we conduct a set of systematic molecular dynamics simulations to investigate the potential of two-dimensional graphene-like channels under extreme confinement to achieve efficient oil-water separation. Effective modulation of the wetting characteristics of graphene-like surfaces juxtaposed with unconventional flow behavior at the nanoscale unveils differential interaction of water and oil molecules towards the wall, thereby resulting in distinct separation zones for varying compositions of the oil-water mixture. Such separation zones have been observed to be highly correlated with mixture temperature, which provides effective separation pathways across diverse environmental conditions. Our study offers a paradigm shift in oil-water separation strategies, which not only provides deeper insights into the equilibrium and dynamic behavior of a two-phase mixture but also holds immense implications for the development of smart, wettability-based oil separation devices.

Estimating water transport in carbon nanotubes: a critical review and inclusion of scale effects

The quasi-frictionless water flow across graphitic surfaces offers vast opportunities for a wide range of applications from biomedical science to energy. However, the conflicting experimental results impede a clear understanding of the transport mechanism and desired flow control. Existing literature proposes numerous modifications and updated boundary conditions to extend classical hydrodynamic theories for nanoflows, yet a consensus or definitive conclusion remains elusive. This study presents a critical review of the proposed modifications of the pressure driven flow or the Hagen-Poiseuille (HP) equations to estimate the flow enhancement through carbon nanotubes (CNTs). For such a case, we performed (semi-)classical molecular dynamics simulations of water flow in various sizes of CNTs, applied the different forms of boundary definitions from the literature, and derived HP equation models by implementing these modifications. By aggregating seven distinct experimental datasets, we tested various flow enhancement models against our measurements. Our findings indicate that including the interfacial layering-based dynamic slip-definition in the proposed HP equations yields accurate estimations. While considering interfacial viscosity predicts the individual CNT experiments well, using the experimental viscosity yields better estimations of measurements for the water flow enhancement through membranes of CNTs. This critical review testing existing literature demonstrates how to refine continuum fluid mechanics to predict water flow enhancement at the nanoscale providing holistic multiscale modeling.

Ultrahigh-Flux Water Nanopumps Generated by Asymmetric Terahertz Absorption

Controlling active transport of water through membrane channels is essential for advanced nanofluidic devices. Despite advancements in water nanopump design using techniques like short-range invasion and subnanometer-level control, challenges remain facilely and remotely realizing massive waters active transport. Herein, using molecular dynamic simulations, we propose an ultrahigh-flux nanopump, powered by frequency-specific terahertz stimulation, capable of unidirectionally transporting massive water through asymmetric-wettability membrane channels at room temperature without any external pressure. The key physics behind this terahertz-powered water nanopump is revealed to be the energy flow resulting from the asymmetric optical absorption of water.

Understanding Water Diffusion Behaviors in Epoxy Resin through Molecular Simulations and Experiments

The unclear understanding of the water diffusion behavior posts a big challenge to the manipulation of water absorption properties in epoxy resins. Herein, we investigated the water diffusion behavior and its relationship with molecule structures inside an epoxy resin mainly by the nonequilibrium molecular dynamics and experiments. It is found that at the initial rapid water absorption stage, bound water and free water both contribute, while at the later slow water absorption stage, free water plays a dominant role. The observed evolution of free water and bound water cannot be explained by the traditional Langmuir model. In addition, molecule polarity, free volume, and segment mobility can all influence the water diffusion process. Hence, the epoxy resin with low polarity and high molecular segment mobility is endowed with higher diffusion coefficients. The saturated water absorption content is almost dependent on the polarity. The understanding of how water diffuses and what decides the diffusion process is critical to the rational design of molecule structures for improving the water resistance in epoxy resin.

The validity of the continuum modeling limit in a single pore flows to the molecular scale

The discrete characteristics of molecules become dominant in the molecular regime when the surface-to-volume ratio becomes very high. Using the well-established continuum approach is questionable due to this dominant behavior. Due to the lack of perfect modeling of such a small-scale system, the experimentalist must rely on the trial and error method. Here we analyze the water transport mechanism through a nanoporous graphene membrane at the molecular level by adopting the classical molecular dynamics (MD) simulation. The results for SPC/E water molecules were compared with those obtained for liquid argon atoms and continuum Sampson's equation predictions. We find that the effect of local variants such as density layering, interatomic forces, slip velocity, and geometric boundary conditions become exponentially dominant with decreasing nanopore size. Consequently, the continuum assumptions break down at 1.5 nm pore diameter due to neglecting the dominant local properties.

Coupling solute interactions with functionalized graphene membranes: towards facile membrane-level engineering

Optimizing ion transport through nanoporous graphene membranes with intricate engineering at nanoscale levels finds applications ranging from ion segregation to desalination. Such membrane-level engineering often requires futuristic and state-of-the-art micro- and nanofabrication infrastructure making it less accessible to widespread applications. In this study, the effective membrane pore size is modulated using macroscopic membrane functionalization, which, when combined with the solute concentration, can prove to be facile nanoscale engineering towards achieving selectivity. By performing robust molecular dynamics (MD) simulations of aqueous NaCl solution through a nanoporous graphene membrane, we demonstrate that varying membrane wettability influences the structural organization of ions and water molecules both in the vicinity and inside the nanopore, which is manifested in the form of altered permeation characteristics. Moreover, the disparate solvation characteristics of the ionic species in conjunction with the variable van der Waals interactive forces affect the ion-selective nature (Cl^- over Na⁺) of the membrane. The relative hydrophilization, resulting from the effective functionalization of the nanoporous graphene membrane, not only allows greater control over the permeation characteristics of ions and water molecules mediated by an altered depletion ratio but also gives rise to the ion-selective nature of the membrane, thus providing a sound understanding of the transport properties of ion-water solutions through nanoporous materials.

Unraveling the Molecular Interface and Boundary Problems in an Electrical Double Layer and Electroosmotic Flow

In a nanofluidic system, the electroosmotic flow (EOF) is a complex fluid transport mechanism, where the formation of an electrical double layer (EDL) occurs ubiquitously at the dissimilar atomic interface. Several studies have suggested various interface boundaries to calculate the EDL thickness. However, the physical origin of the interface boundary and its effects on the flow properties is not yet clearly understood. Combining the theoretical framework and molecular dynamics (MD) simulations, we show the effects of different interfacial boundaries on the EDL thickness and EOF characteristics. Implemented interface boundaries exhibit the EDL thickness-boundary relation, i.e., the EDL thickness from MD simulations shows the tendency of converging toward the continuum approximation. Furthermore, inserting these values of EDL thicknesses into the continuum equation shows the convergence of flow transition of the molecular state to a neutral from an electrical violation phase, which takes a parabolic to plug-like shape in the velocity profile. Different interface boundaries also affect the hydrodynamic properties (viscosity and electroviscosity) of EOF, which varies from the bulk to interface region, as well as the fluid flow. Therefore, we can infer that, at the molecular level, the dissimilar atomic boundary and hydrodynamic properties dominate the electrokinetic flow. Our simulation results and theoretical model provide fundamental insightful information and guidelines for the EOF study based on the atomic interface and dynamic structure-based hydrodynamic property.

Pore Size Dependence of Permeability in Bicontinuous Nanoporous Media

In this work, we employ many-body dissipative particle dynamics (mDPD) simulations to investigate the fluid flow process through bicontinuous nanoporous media, which are representative models for a broad class of nanoporous materials. The mDPD formulation includes attractive and repulsive interactions describing accurately fluid-fluid and fluid-solid interactions. As a mesoscale simulation method, mDPD can bridge the length and time scale gap between continuum and atomistic simulations. The bicontinuous nanoporous models are constructed considering a defined morphology, the porosity level, and varying pore sizes in the range from 3.41 to 13.63 nm. All models have a 0.65 porosity level and the same topology. The models provide a stochastic description of the morphology and pore size distribution and allow for a direct investigation of the dependence of permeability on the average pore size. The stationary nanoporous models are filled with fluid particles, and flow is induced by the action of confining pistons. Simulation results, obtained by imposing different pressure differences on the surfaces of the nanoporous media, indicate a linear pressure drop within the nanoporous model. Regardless of the complexities and different scales of the porous media considered, the steady-state fluid flow through the nanoporous models is proportional to the pressure gradient applied, in agreement with Darcy's law. The calculated pore size dependence of permeability is well described by the Hagen-Poiseuille law, considering a single shape correction factor that accounts for the flow resistance due to the complex nanoporous morphology. This work highlights the effect of the average pore size of a complex stochastic bicontinuous nanoporous medium on fluid properties. The results indicate rather a relatively simple dependence of permeability on the average pore size. The novel method we employ to generate the stochastic bicontinuous nanoporous structure allows the control of different geometric features that can be explored in future studies.