Interlink between Abnormal Water Imbibition in Hydrophilic and Rapid Flow in Hydrophobic Nanochannels.

Nanoscale extension and refinement of the Lucas-Washburn model is presented with a detailed analysis of recent experimental data and extensive molecular dynamics simulations to investigate rapid water flow and water imbibition within nanocapillaries. Through a comparative analysis of capillary rise in hydrophilic nanochannels, an unexpected reversal of the anticipated trend, with an abnormal peak, of imbibition length below the size of 3 nm was discovered in hydrophilic nanochannels, surprisingly sharing the same physical origin as the well-known peak observed in flow rate within hydrophobic nanochannels. The extended imbibition model is applicable across diverse spatiotemporal scales and validated against simulation results and existing experimental data for both hydrophilic and hydrophobic nanochannels.

Predictive Value of a Novel Baseline Diffusion-Weighted Imaging Posterior Circulation Score in Endovascular Treatment of Patients with Acute Vertebrobasilar Occlusion.

RATIONALE AND OBJECTIVES: To investigate the predictive value of a novel posterior circulation score (novel-PC score) based on baseline posterior circulation diffusion-weighted imaging (DWI) for functional independence after endovascular treatment (EVT) in patients with acute vertebral-basilar artery occlusion (VBAO). MATERIALS AND METHODS: The baseline DWI brain stem score (BSS), posterior circulation Alberta Stroke Program Early CT Score (pc-ASPECTS), and the novel-PC score were evaluated separately. A modified Rankin scale (mRS) ≤2 at 90 days was defined as a prognostic indicator of functional independence. Modified Thrombolysis in Cerebral Infarction grade 2b or 3 was defined as surgical success. RESULTS: A total of 64 eligible patients were enrolled and divided into good and poor functional prognosis groups based on the mRS. The novel-PC score, BSS, and pc-ASPECTS (all P ≤ .001) were significantly better in the good functional prognosis group. The novel-PC score had a higher predictive value than BSS and pc-ASPECTS for 90-day functional independence (area under the receiver operating characteristic curve, 0.87 vs. 0.73 vs. 0.71; P < .05). Univariate binary logistic regression analysis showed that age (P = .006), Posterior National Institutes of Health Stroke Scale ≤18 (P < .001), BSS ≤2 (P = .008), pc-ASPECTS >7 (P = .002), and novel-PC score ≤5 (P = .001) were independently associated with function. CONCLUSION: Our novel-PC score, based on DWI, can independently predict functional prognosis in patients with acute VBAO after EVT. CLINICAL RELEVANCE: The novel-PC score based on baseline DWI was shown to be an independent predictor of function in patients with acute BVAO who are treated with EVT.

Magnetic resonance imaging indicators for neurological outcome after surgery in patients with intramedullary spinal ependymomas.

ABSTRACT: This is a retrospective study. The aim of this study was to determine the indicators of neurological outcome after surgery in patients with intramedullary spinal ependymomas by using magnetic resonance imaging (MRI).A total of 106 consecutive patients (mean age: 42.4 ±â€Š1.3 years; 52.8% male) diagnosed with intramedullary spinal ependymomas were retrospectively recruited. All patients underwent spine MRI and subsequent surgical resection for the spinal tumors. Data regarding clinical symptoms and pathological grades of tumors were collected from clinical records. The McCormick score was used for grading patients' neurological status before and after surgery at 12 months. Good outcome was defined as stable McCormick score (McC) score (no change of McC score between preoperation and post-operation at 12 months) or improvement in McC score (post-operative McC score at 12 months < preoperative McC score). Poor outcome was determined when there was an increase in McC score at 12 months after surgery. The MRI characteristics of spinal ependymomas between patients with good and poor neurological outcomes were compared. Logistic regression was performed to assess the association between MRI characteristics of tumors and post-operative neurological outcomes.Patients with poor neurological outcomes had larger longitudinal length (4.7 ±â€Š0.5 vs 3.3 ±â€Š0.2, P = .004) and higher enhancement signal-to-noise-ratio (SNR) (102.4 ±â€Š12.3 vs 72.8 ±â€Š4.6, P = .022) than those with good neurological outcomes. After adjusting for confounding factors, longitudinal length (OR, 0.768; 95% CI, 0.604-0.976; P = .031) and enhancement SNR (OR, 0.988; 95% CI, 0.978-0.999; P = .026) of spinal ependymomas were significantly associated with poor neurological prognosis.The longitudinal length of tumor and enhancement SNR on T1-weighted images are independently associated with neurological outcome after surgery.

A hydrogen bond-modulated soft nanoscale water channel for ion transport through liquid-liquid interfaces.

Ion transport through interfaces is of ubiquitous importance in many fields such as electrochemistry, emulsion stabilization, phase transfer catalysis, liquid-liquid extraction and enhanced oil recovery. However, the knowledge of interfacial structures that significantly affect ion transport through liquid-liquid interfaces is still lacking due to the difficulty of observing nanoscale interfaces. We studied here the evolution of interfacial structures during ion transport through the decane-water interface under different ionic concentrations and external forces using molecular dynamics simulations. The roles of hydrogen bonds in ion transport through interfaces are revealed. We identified a soft nanoscale channel during ion transport through liquid-liquid interfaces and the decane phase under specific external force. The stability of the water channel and the ion transport velocity both increase with ionic concentration due to the layered ordering structures of the water near the channel surface. We observed that the stability and connectivity of the water channel in the decane phase are remarkably improved both by the high increase of the number of hydrogen bonds in the water channel with increasing ionic concentration, and by the conformational change in water molecules near the water channel surface. Our discovery of a soft nanoscale water channel by molecular simulations implies that there is a potential stable passage for ion transport through liquid-liquid interfaces.

Surfactant desorption and scission free energies for cylindrical and spherical micelles from umbrella-sampling molecular dynamics simulations.

HYPOTHESIS: The free energies associated with adsorption/desorption of individual surfactants from micelles and the fusion/scission of long micelles can be used to estimate the rate constants for micellar kinetics as functions of surfactant and salt concentration. EXPERIMENTS: We compute the escape free energies △Gesc of surfactant from micelles and the scission free energies △Gsciss of long micelles from coarse-grained molecular dynamics simulations coupled with umbrella sampling, for micelles of both sodium dodecylsulfate (SDS) in sodium chloride (NaCl) and cetyltrimethylammonium chloride (CTAC) in sodium salicylate (NaSal). FINDINGS: For spherical micelles, △Gesc values have maxima at certain aggregation numbers, and at salt-to-surfactant molar concentration ratios R near unity, consistent with experiments. For cylindrical micelles, SDS/NaCl shows a minimum, and CTAC/NaSal a maximum in △Gesc, both at R ~ 0.7, while △Gsciss of CTAC micelles also peaks at around R ~ 0.7 and that of SDS micelles increases monotonically with R. We explain the non-monotonic dependence of escape and scission free energies on R by a combination of electrostatic screening and the decrease of micelle radius with increasing R. Transitions from predominantly spherical to cylindrical micelles, and between adsorption/desorption and fusion/scission kinetics with changing salt concentration can be inferred from the free energies for CTAC/NaSal.

Extending the Classical Continuum Theory to Describe Water Flow through Two-Dimensional Nanopores.

Water flow through two-dimensional nanopores has attracted significant attention owing to the promising water purification technology based on atomically thick membranes. However, the theoretical description of water flow in nanopores based on the classical continuum theory is very challenging owing to the pronounced entrance/exit effects. Here, we extend the classical Hagen-Poiseuille equation for describing the relationship between flow rate and pressure loss in laminar tube flow to two-dimensional nanopores. A totally theoretical model is established by appropriately considering the velocity slip on pore surfaces both in the friction pressure loss and entrance/exit pressure loss. Based on molecular dynamics simulations of water flow through graphene nanopores, it is shown that the model can not only well predict the overall flow rate but also give a good estimation of the velocity profiles. As the pore radius and length increase, the model can reduce to the equations applicable to the fluid flow in infinitely/finitely long nanotubes, thin orifices, and macroscale tubes, showing an accurate prediction of the existing experimental and simulation data of the water flow through nanotubes and nanopores in the literature. Namely, the presented model is a unified model that can uniformly describe the fluid flow from nanoscales to macroscales by modifying the classical continuum theory.

Theoretical description of molecular permeation via surface diffusion through graphene nanopores.

We establish a theoretical model to describe the surface molecular permeation through two-dimensional graphene nanopores based on the surface diffusion equation and Fick's law. The model is established by considering molecular adsorption and desorption from the surface adsorption layer and the molecular diffusion and concentration gradient on the graphene surface. By comparing with the surface flux obtained from molecular dynamics simulations, it is shown that the model can predict well the overall permeation flux especially for strongly adsorbed molecules (i.e. CO2 and H2S) on graphene surfaces. Although good agreement between the theoretical and simulated density distribution is hard to achieve owing to the large uncertainty in the calculation of surface diffusion coefficients based on the Einstein equation, the model itself is very competent to describe the surface molecular permeation both from the aspects of the overall permeation flux and detailed density distribution. This model is believed to supplement the theoretical description of molecular permeation through graphene nanopores and provide a good reference for the description of mass transport through two-dimensional porous materials.

Wall friction should be decoupled from fluid viscosity for the prediction of nanoscale flow.

The accurate determination of fluid viscosity based on the microscopic information of molecules is very crucial for the prediction of nanoscale flow. Despite the challenge of this problem, researchers have done a lot of meaningful work and developed several distinctive methods. However, one of the common approaches to calculate the fluid viscosity is using the Green-Kubo formula by considering all the fluid molecules in nanospace, inevitably causing the involvement of the frictional interaction between fluid and the wall into the fluid viscosity. This practice is certainly not appropriate because viscosity is essentially related only to the interactions among fluid molecules. Here, we clarify that the wall friction should be decoupled from fluid viscosity by distinguishing the frictional region and the viscous region for the accurate prediction of nanoscale flow. By comparing the fluid viscosities calculated from the Green-Kubo formula in the whole region and viscous region and the viscosity obtained from the velocity profile through the Hagen-Poiseuille equation, it is found that only the calculated viscosity in the viscous region agrees well with the viscosity from the velocity profile. To demonstrate the applicability of this clarification, the Lennard-Jones fluid and water confined between Lennard-Jones, graphene, and silica walls, even with different fluid-wall interactions, are extensively tested. This work clearly defines the viscosity of fluids at nanoscales from the inherent nature of physics, aiming at the accurate prediction of nanoscale flow from the classical continuum hydrodynamic theory.

Nanoconfined Fluids: What Can We Expect from Them?

Nanoconfined fluids (NCFs), which are confined in nanospaces, exhibit distinctive nanoscale effects, including surface effects, small-size effects, quantum effects, and others. The continuous medium hypothesis in fluid mechanics is not valid in this context because of the comparable characteristic length of spaces and molecular mean free path, and accordingly, the classical continuum theories developed for the bulk fluids usually cannot describe the mass and energy transport of NCFs. In this Perspective, we summarize the nanoscale effects on the thermodynamics, mass transport, flow dynamics, heat transfer, phase change, and energy transport of NCFs and highlight the related representative works. The applications of NCFs in the fields of membrane separation, oil and gas production, energy harvesting and storage, and biological engineering are especially indicated. Currently, the theoretical description framework of NCFs is still missing, and it is expected that this framework can be established by adopting the classical continuum theories with the consideration of nanoscale effects.

Evidence for water ridges at oil-water interfaces: implications for ion transport.

Understanding ion transport across interfaces is of fundamental importance in many processes such as liquid-liquid extraction, phase transfer catalysis, enhanced oil recovery and emulsion stabilisation. However, the factors that control ion transport across interfaces are poorly known due to a lack of knowledge of structural changes at interfaces. We studied here the effects of ionic concentration and external force on the transport of ions across the decane-water interface using classical molecular dynamics simulations. The results show that the evolution of interfacial structures during ion transfer across the interface is controlled by hydrogen bonding and ionic interactions at the interface. We also identified a new mode of ion transfer across the interface at low ionic concentrations, involving a 'water ridge', rather that the classical 'water finger'. In the water ridge mode, hydrogen bonds are not broken due to low ion levels, and the water ridge induces gradual interface deformation. Whereas, at high ionic concentrations, hydrogen bonds are broken by the strong ion electrostatic repulsion, thus inducing the formation of a water finger. We also found that the variation of the Gibbs free energy during ion transfer is directly relevant to the ionic concentration. The water ridge at low ionic concentrations, which displaces more water molecules towards the decane phase, induces less free energy variation than the water finger at high ionic concentrations.

Effects of Molecular Chain Length on the Contact Line Movement in Water/n-Alkane/Solid Systems.

The movement of the contact line in liquid-liquid-solid systems is a major phenomenon in natural and industrial processes. In particular, n-alkanes are widely occurring in the oil, soil pollution, and chemical industries, yet there is little knowledge on the effects of molecular chain length on the contact line movement. Here, we studied the effects of molecular chain length on the contact line movement in water/n-alkane/solid systems with different surface wettabilities. We used n-heptane (C7), n-decane (C10), and n-hexadecane (C16) as alkanes and α-quartz as the solid surface. We calculated the time-variation contact line moving velocity and also analyzed the jump frequency and the mean distance of the molecular displacement occurring within the contact line zone by molecular-kinetic theory. Molecular dynamics simulation results show that the contact line velocity decreases with increasing the chain length, originally caused by the decreasing the jump frequency and mean distance. These variations with the molecular chain length are related to the more torsions and deformations of the molecules with a longer chain length. In addition, the moving mechanism of the contact line on the same solid surface does not change at different molecular chain lengths, implying that the moving mechanism mainly depends on the three-phase wettability.

Selective Molecular Sieving through a Large Graphene Nanopore with Surface Charges.

The precise control of the pore sizes at an atomic level has proved to be the biggest challenge of all for nanoporous graphene membranes for gas separation. Here, we propose a simple method to realize the selective molecular sieving through originally nonselective graphene nanopores by adding charges on the graphene surfaces. Molecular dynamic simulations show that the CO2/N2 selectivity of the graphene nanopore with a diameter of 0.52 nm increases up to 22.78 for a surface charge density of only -5.934 e/nm2. The selectivity improvement is related to the distinctive adsorption intensities of CO2 and N2 molecules on the charge-loaded graphene surfaces. This work points toward a promising road to tune the selectivity of graphene nanopores and therefore promotes the realization of porous graphene membranes and other two-dimensional porous membranes by accepting the pores with a wide size distribution and reducing the requirements in the control of pore sizes.

Release of a trapped droplet in a single micro pore throat.

The critical condition (i.e., critical capillary number Cac) for the release of trapped droplets is of practical importance in enhanced oil recovery and droplet microfluidics. In a recent study, Cac was obtained for a long droplet with a size much larger than the channel size. However, in real applications, trapped droplets are often finite with a size comparable to the channel, in which capillary and hydrostatic pressures alternate significantly. We hypothesize that Cac of finite droplets has a discrepancy from that of long droplets. Microfluidic experiments were performed to obtain the critical condition for the release of a finite droplet trapped in a single pore throat. A theoretical prediction via analyzing capillary and hydrostatic pressures was derived for Cac of both finite and long droplets. We find that Cac strongly depends on the droplet-to-channel size ratio (i.e., the droplet-to-convergent channel length ratio L/Lc). In particular, Cac increases with L/Lc for finite droplets (i.e., L/Lc < 1) but shows an opposite tendency for long droplets (i.e., L/Lc > 1), as demonstrated in previous studies. Via theoretical analysis, we established a predictive criterion for Cac versus L/Lc, and this criterion quantitatively agrees well with experimental data for both finite and long droplets.

Moving mechanisms of the three-phase contact line in a water-decane-silica system.

The movement of the three-phase contact line with chain molecules in the liquid phase displays more complex mechanisms compared to those in the usual liquid-liquid-solid systems and even to the gas-liquid-solid systems controlled by the traditional single-molecule adsorption-desorption mechanisms. By introducing decane molecules with chain structures, we demonstrate from molecular dynamics insights that the moving mechanism of the contact line in a water-decane-silica system is totally different from traditional mechanisms. Three different wettability-related moving mechanisms including "Roll up", "Piston" and "Shear" are revealed corresponding to the hydrophilic, intermediate and hydrophobic three-phase wettability, respectively. In the "Roll up" mechanism, the decane molecules are rolled up by the competitively adsorbed water molecules and then move forward under the driving force; when the "Piston" mechanism happens, the decane molecules are pushed by the piston-like water phase owing to the comparable adsorption interactions of the two liquids on the solid surface; in the "Shear" mechanism, the contact line is hard to drive due to the stronger decane-silica interactions but the decane molecules far away from the solid surface will move forward. Besides, the time-averaged velocity of the moving contact line is greatly related to the moving mechanisms. For the "Roll up" mechanism, the contact line velocity increases first and then reaches a steady value; for the "Piston" mechanism, the contact line velocity has a maximum value at the start-up stage and then decreases to a stable value; for the "Shear" mechanism, the contact line velocity fluctuates around zero due to the thermal fluctuation of the molecules. Additionally, the mean distance from Molecular Kinetics Theory increases with decreasing hydrophilicity and the displacement frequency in "Roll up" mechanism is 2 orders of magnitude higher than that in the "Piston" mechanism, further demonstrating the different moving mechanisms from a quantitative point of view.

Experiment and Numerical Simulation on Gas-Liquid Annular Flow through a Cone Sensor.

The cone meter has been paid increasing attention in wet gas measurement, due to its distinct advantages. However, the cone sensor, which is an essential primary element of the cone meter, plays a role in the measurement of wet gas flow that is important, but not fully understood. In this article, we investigate the gas-liquid annular flow through a cone sensor by experiment and numerical simulation. Emphasis is put on the influences of pressure recovery characteristics and flow structure, and how they are affected by the cone sensor. The results show that the vortex length is shortened in gas-liquid annular flow, compared with that in single-phase gas flow. The pressure recovery length is closely related with the vortex length, and shorter vortex length leads to shorter pressure recovery length. The gas-liquid distribution suggests that flow around the apex of back-cone is very stable, little liquid is entrained into the vortex, and no liquid appears around the low pressure tapping, which makes a more stable pressure at the apex of cone sensor feasible. This finding highlights the importance of obtaining the low pressure from the back-cone apex, which should be recommended in the multiphase flow measurement. Our results may help to guide the optimization of the cone sensor structure in the wet gas measurement.

Nanoparticle-induced ion-sensitive reduction in decane-water interfacial tension.

The synergistic effect of ions and nanoparticles on the interfacial tension is of great significance for extensive applications in interface-related industrial processes. However, its mechanisms are still unclear owing to a lack of understanding on the interaction between nanoparticles/ions at the interface. Here, we employ the molecular dynamics method to explore the synergistic effect of ions and nanoparticles on reducing the decane-water interfacial tension and reveal the dominant role of the three-phase contact angle and the interaction between nanoparticles. The results show that the reduction of interfacial tension is sensitive to cation species and temperature. The stronger hydration of cations induces an increased three-phase contact angle, weakening the interaction between nanoparticles and water molecules at the interface. Hence, the virial term of interfacial tension decreases. Meanwhile, the potential of mean force between nanoparticles at the interface indicates that the order of interaction strength between nanoparticles for different cations is Ca2+ > Mg2+ > Na+. The strong interaction between nanoparticles restricts the motion of nanoparticles and water molecules at the interface, inducing a reduced kinetic energy term of interfacial tension. Therefore, the interfacial tension decreases after adding the nanoparticles. Besides, as temperature rises, the difference in the adsorption ability of nanoparticles on water molecules causes a falling interfacial tension with a characteristic stage.

Oil Contact Angles in a Water-Decane-Silicon Dioxide System: Effects of Surface Charge.

Oil wettability in the water-oil-rock systems is very sensitive to the evolution of surface charges on the rock surfaces induced by the adsorption of ions and other chemical agents in water flooding. Through a set of large-scale molecular dynamics simulations, we reveal the effects of surface charge on the oil contact angles in an ideal water-decane-silicon dioxide system. The results show that the contact angles of oil nano-droplets have a great dependence on the surface charges. As the surface charge density exceeds a critical value of 0.992 e/nm2, the contact angle reaches up to 78.8° and the water-wet state is very apparent. The variation of contact angles can be confirmed from the number density distributions of oil molecules. With increasing the surface charge density, the adsorption of oil molecules weakens and the contact areas between nano-droplets and silicon dioxide surface are reduced. In addition, the number density distributions, RDF distributions, and molecular orientations indicate that the oil molecules are adsorbed on the silicon dioxide surface layer-by-layer with an orientation parallel to the surface. However, the layered structure of oil molecules near the silicon dioxide surface becomes more and more obscure at higher surface charge densities.

Ionic hydration-induced evolution of decane-water interfacial tension.

Building a connection between the variations in interfacial tension and the microstructure of the oil-water interface is still very challenging. Here, we employ a molecular dynamics method to study the effect of monovalent ions on the decane-water interfacial tension and reveal the relationship between ionic hydration and the variation of interfacial tension. Our results indicate that interfacial tension presents a non-monotonic dependence on the ionic concentrations owing to the distinctive adsorption characteristics of ions. At low ionic concentrations, the hydration of the discrete ions at the interface causes an enhancement in the virial term of the interfacial tension, resulting in an increase of the interfacial tension with increasing ionic concentrations. At high ionic concentrations, the ion pairs at the interface weaken the ionic hydration, thus the virial term of the interfacial tension decreases and the interfacial tension decreases slightly. In addition, the kinetic energy term of interfacial tension increases only with increasing temperature, while the virial term decreases with an increase in either temperature or pressure on account of the weakening ionic hydration; therefore, the increase of temperature and pressure induces different degrees of the decrease in the interfacial tension owing to the major contribution of the virial term, particularly at high ionic concentrations.

Gas diffusion on graphene surfaces.

Graphene provides a possibility where gas adsorption energy is comparable with molecular collision energy for physically adsorbed gases, resulting in the incompetence of the traditional hopping model to describe graphene-related surface diffusion phenomena. By calculating surface diffusion coefficients based on the Einstein equation, we exactly demonstrate that the gas diffusion on a graphene surface is a two-dimensional gas behavior mainly controlled by the collisions between adsorbed molecules. The surface diffusion on the graphene film just follows the bulk diffusion qualitatively, namely the diffusion coefficients decrease with increasing gas pressure. Quantitatively, the surface diffusion coefficients are lower than the bulk diffusion coefficients, predicted using the hard sphere model, owing to the restriction of graphene films. The reduction in diffusion coefficient is related to the simultaneously suppressed average frequency of molecular collisions and the average travelling distance between successive collisions. In addition, a lower diffusion coefficient on a hydrogen-functionalized graphene surface is identified, caused by the blocking effects of chemical functional groups.

Molecular Dynamics Study on the Effect of Surface Hydroxyl Groups on Three-Phase Wettability in Oil-Water-Graphite Systems.

In this paper, a hydroxylated graphite surface is generated as a hydrophilic oleophobic material for the application of oil-water separation, and the effects of hydroxyl density on the three-phase wettability are studied in oil-water-graphite systems. We analyze the adsorption of water molecules on the hydroxylated surfaces and obtain the relationship between water-oil-solid interfacial properties and the hydroxyl density, which results from the synthetic effects of the orientation of molecules and hydrogen bonds. With the increase of hydroxyl density, the water-solid contact angle first decreases rapidly, and then remains constant. The density of the hydrogen bond formed between hydroxyls and water molecules in the adsorption layer can explain the regularity of the three-phase wettability. The orientation of the water molecules in the adsorption layer shows insignificant variation, owing to the hydrogen bond network formed between the water molecules; thus, little change is observed in the hydrogen bond density in the adsorption layer.